DOCS/client_api_examples: add an alternative qml example

This one avoids use of a FBO. It's less flexible, because it uses works
around the whole QML rendering API. It seems to be the only way to get
OpenGL rendering without any indirections, though.

Parts of this example were insipired by Qt's "Squircle" example.

Also add a README file with a short description of each example, to
reduce the initial confusing.

DOCS/client_api_examples/README.md

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+Client API examples
+===================
+
+All these examples use the mpv client API through libmpv.
+
+cocoa
+-----
+
+Shows how to embed the mpv video window in Objective-C/Cocoa.
+
+qml
+---
+
+Shows how to use mpv's OpenGL video renderer in QtQuick2 with QML.
+
+qml_direct
+----------
+
+Alternative example, which typically avoids a FBO indirection. Might be
+slightly faster, but is less flexible and harder to use.
+
+qt
+--
+
+Shows how to embed the mpv video window in Qt (using normal desktop widgets).
+
+simple
+------
+
+Very primitive terminal-only example. Shows some most basic API usage.

DOCS/client_api_examples/qml_direct/main.cpp

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+#include "main.h"
+
+#include <stdexcept>
+
+#include <QObject>
+#include <QtGlobal>
+#include <QOpenGLContext>
+
+#include <QGuiApplication>
+#include <QtQuick/QQuickWindow>
+#include <QtQuick/QQuickView>
+
+static void *get_proc_address(void *ctx, const char *name) {
+    (void)ctx;
+    QOpenGLContext *glctx = QOpenGLContext::currentContext();
+    if (!glctx)
+        return NULL;
+    return (void *)glctx->getProcAddress(QByteArray(name));
+}
+
+MpvRenderer::MpvRenderer(mpv::qt::Handle a_mpv, mpv_opengl_cb_context *a_mpv_gl)
+    : mpv(a_mpv), mpv_gl(a_mpv_gl), window(0)
+{
+    int r = mpv_opengl_cb_init_gl(mpv_gl, NULL, get_proc_address, NULL);
+    if (r < 0)
+        throw std::runtime_error("could not initialize OpenGL");
+}
+
+MpvRenderer::~MpvRenderer()
+{
+    // Until this call is done, we need to make sure the player remains
+    // alive. This is done implicitly with the mpv::qt::Handle instance
+    // in this class.
+    mpv_opengl_cb_uninit_gl(mpv_gl);
+}
+
+void MpvRenderer::paint()
+{
+    window->resetOpenGLState();
+
+    // Render to the whole window.
+    QSize s = window->size() * window->devicePixelRatio();
+    int vp[4] = {0, 0, s.width(), -s.height()};
+
+    // This uses 0 as framebuffer, which indicates that mpv will render directly
+    // to the frontbuffer. Note that mpv will always switch framebuffers
+    // explicitly. Some QWindow setups (such as using QQuickWidget) actually
+    // want you to render into a FBO in the beforeRendering() signal, and this
+    // code won't work there.
+    mpv_opengl_cb_render(mpv_gl, 0, vp);
+
+    window->resetOpenGLState();
+}
+
+MpvObject::MpvObject(QQuickItem * parent)
+    : QQuickItem(parent), mpv_gl(0), renderer(0)
+{
+    mpv = mpv::qt::Handle::FromRawHandle(mpv_create());
+    if (!mpv)
+        throw std::runtime_error("could not create mpv context");
+
+    mpv_set_option_string(mpv, "terminal", "yes");
+    mpv_set_option_string(mpv, "msg-level", "all=v");
+
+    if (mpv_initialize(mpv) < 0)
+        throw std::runtime_error("could not initialize mpv context");
+
+    // Make use of the MPV_SUB_API_OPENGL_CB API.
+    mpv::qt::set_option_variant(mpv, "vo", "opengl-cb");
+
+    // Setup the callback that will make QtQuick update and redraw if there
+    // is a new video frame. Use a queued connection: this makes sure the
+    // doUpdate() function is run on the GUI thread.
+    mpv_gl = (mpv_opengl_cb_context *)mpv_get_sub_api(mpv, MPV_SUB_API_OPENGL_CB);
+    if (!mpv_gl)
+        throw std::runtime_error("OpenGL not compiled in");
+    mpv_opengl_cb_set_update_callback(mpv_gl, MpvObject::on_update, (void *)this);
+    connect(this, &MpvObject::onUpdate, this, &MpvObject::doUpdate,
+            Qt::QueuedConnection);
+
+    connect(this, &QQuickItem::windowChanged,
+            this, &MpvObject::handleWindowChanged);
+}
+
+MpvObject::~MpvObject()
+{
+    if (mpv_gl)
+        mpv_opengl_cb_set_update_callback(mpv_gl, NULL, NULL);
+}
+
+void MpvObject::handleWindowChanged(QQuickWindow *win)
+{
+    if (!win)
+        return;
+    connect(win, &QQuickWindow::beforeSynchronizing,
+            this, &MpvObject::sync, Qt::DirectConnection);
+    connect(win, &QQuickWindow::sceneGraphInvalidated,
+            this, &MpvObject::cleanup, Qt::DirectConnection);
+    win->setClearBeforeRendering(false);
+}
+
+void MpvObject::sync()
+{
+    if (!renderer) {
+        renderer = new MpvRenderer(mpv, mpv_gl);
+        connect(window(), &QQuickWindow::beforeRendering,
+                renderer, &MpvRenderer::paint, Qt::DirectConnection);
+    }
+    renderer->window = window();
+}
+
+void MpvObject::cleanup()
+{
+    if (renderer) {
+        delete renderer;
+        renderer = 0;
+    }
+}
+
+void MpvObject::on_update(void *ctx)
+{
+    MpvObject *self = (MpvObject *)ctx;
+    emit self->onUpdate();
+}
+
+// connected to onUpdate(); signal makes sure it runs on the GUI thread
+void MpvObject::doUpdate()
+{
+    window()->update();
+}
+
+void MpvObject::command(const QVariant& params)
+{
+    mpv::qt::command_variant(mpv, params);
+}
+
+int main(int argc, char **argv)
+{
+    QGuiApplication app(argc, argv);
+
+    qmlRegisterType<MpvObject>("mpvtest", 1, 0, "MpvObject");
+
+    QQuickView view;
+    view.setResizeMode(QQuickView::SizeRootObjectToView);
+    view.setSource(QUrl("qrc:///mpvtest/main.qml"));
+    view.show();
+
+    return app.exec();
+}

DOCS/client_api_examples/qml_direct/main.h

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+#ifndef MPVRENDERER_H_
+#define MPVRENDERER_H_
+
+#include <QtQuick/QQuickItem>
+
+#include <mpv/client.h>
+#include <mpv/opengl_cb.h>
+#include <mpv/qthelper.hpp>
+
+class MpvRenderer : public QObject
+{
+    Q_OBJECT
+    mpv::qt::Handle mpv;
+    mpv_opengl_cb_context *mpv_gl;
+    QQuickWindow *window;
+
+    friend class MpvObject;
+public:
+    MpvRenderer(mpv::qt::Handle a_mpv, mpv_opengl_cb_context *a_mpv_gl);
+    virtual ~MpvRenderer();
+public slots:
+    void paint();
+};
+
+class MpvObject : public QQuickItem
+{
+    Q_OBJECT
+
+    mpv::qt::Handle mpv;
+    mpv_opengl_cb_context *mpv_gl;
+    MpvRenderer *renderer;
+
+public:
+    MpvObject(QQuickItem * parent = 0);
+    virtual ~MpvObject();
+public slots:
+    void command(const QVariant& params);
+    void sync();
+    void cleanup();
+signals:
+    void onUpdate();
+private slots:
+    void doUpdate();
+    void handleWindowChanged(QQuickWindow *win);
+private:
+    static void on_update(void *ctx);
+};
+
+#endif

DOCS/client_api_examples/qml_direct/main.qml

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+import QtQuick 2.0
+import QtQuick.Controls 1.0
+
+import mpvtest 1.0
+
+Item {
+    width: 1280
+    height: 720
+
+    MpvObject {
+        id: renderer
+
+        // This object isn't real and not visible; it just renders into the
+        // background of the containing Window.
+        width: 0
+        height: 0
+    }
+
+    MouseArea {
+        anchors.fill: parent
+        onClicked: renderer.command(["loadfile", "../../../test.mkv"])
+    }
+
+    Rectangle {
+        id: labelFrame
+        anchors.margins: -50
+        radius: 5
+        color: "white"
+        border.color: "black"
+        opacity: 0.8
+        anchors.fill: box
+    }
+
+    Row {
+        id: box
+        anchors.bottom: parent.bottom
+        anchors.left: parent.left
+        anchors.right: parent.right
+        anchors.margins: 100
+
+        Text {
+            anchors.margins: 10
+            wrapMode: Text.WordWrap
+            text: "QtQuick and mpv are both rendering stuff.\n
+                   In this example, mpv is always in the background.\n
+                   Click to load ../../../test.mkv"
+        }
+    }
+}

DOCS/client_api_examples/qml_direct/mpvtest.pro

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+QT += qml quick
+
+HEADERS += main.h
+SOURCES += main.cpp
+
+CONFIG += link_pkgconfig debug
+PKGCONFIG += mpv
+
+RESOURCES += mpvtest.qrc
+
+OTHER_FILES += main.qml

DOCS/client_api_examples/qml_direct/mpvtest.qrc

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+<RCC>
+    <qresource prefix="/mpvtest">
+        <file>main.qml</file>
+    </qresource>
+</RCC>