DOCS/client_api_examples: qml: add test for gamma control

Provides a dumb test for the functionality added in the previous commit.

DOCS/client_api_examples/qml/main.qml

@@ -41,18 +41,31 @@ Item {
                    Click to load ../../../test.mkv"
         }
 
-        CheckBox {
-            id: checkbox
-            anchors.margins: 10
-            // Heavily filtered means good, right?
-            text: "Make video looks like on a Smart TV"
-            onClicked: {
-                if (checkbox.checked) {
-                    renderer.command(["vo_cmdline", "lscale=sharpen3:lparam1=1.0"])
-                } else {
-                    renderer.command(["vo_cmdline", ""])
+        // Don't take these controls too seriously. They're for testing.
+        Column {
+            CheckBox {
+                id: checkbox
+                anchors.margins: 10
+                // Heavily filtered means good, right?
+                text: "Make video look like on a Smart TV"
+                onClicked: {
+                    if (checkbox.checked) {
+                        renderer.command(["vo_cmdline", "lscale=sharpen3:lparam1=5.0"])
+                    } else {
+                        renderer.command(["vo_cmdline", ""])
+                    }
                 }
             }
+            Slider {
+                id: slider
+                anchors.margins: 10
+                anchors.left: checkbox.left
+                anchors.right: checkbox.right
+                minimumValue: -100
+                maximumValue:  100
+                value: 0
+                onValueChanged: renderer.setProperty("gamma", slider.value | 0)
+            }
         }
     }
 }

DOCS/client_api_examples/qml/mpvrenderer.cpp

@@ -100,10 +100,14 @@ void MpvObject::doUpdate()
 
 void MpvObject::command(const QVariant& params)
 {
-    //mpv_command_string(mpv, s.toUtf8().data());
     mpv::qt::command_variant(mpv, params);
 }
 
+void MpvObject::setProperty(const QString& name, const QVariant& value)
+{
+    mpv::qt::set_property_variant(mpv, name, value);
+}
+
 QQuickFramebufferObject::Renderer *MpvObject::createRenderer() const
 {
     window()->setPersistentOpenGLContext(true);

DOCS/client_api_examples/qml/mpvrenderer.h

@@ -24,6 +24,7 @@ public:
     virtual Renderer *createRenderer() const;
 public slots:
     void command(const QVariant& params);
+    void setProperty(const QString& name, const QVariant& value);
 signals:
     void onUpdate();
 private slots: