options: add --native-touch option

For platforms which send emulated mouse inputs for touch-unaware clients,
such as Win32 and X11, this option enables the native multitouch handling
and disables emulated mouse inputs. This enables interested scripts to
handle multitouch events while keeping compatibility with touch-unaware
scripts.

DOCS/interface-changes/native-touch.txt

@@ -0,0 +1 @@
+add --native-touch option

DOCS/man/options.rst

@@ -4101,6 +4101,17 @@ Input
     Whether this applies depends on the VO backend and how it handles
     keyboard input. Does not apply to terminal input.
 
+``--native-touch=<yes|no>``
+    (Windows only)
+    For platforms which send emulated mouse inputs for touch-unaware clients,
+    such as Windows, use system native touch events, instead of receiving them
+    as emulated mouse events (default: no). This is required for multi-touch
+    support for these platforms.
+
+    Note that this option has no effect on other platforms: either native touch
+    is not supported by mpv, or the platform does not give an option to receive
+    emulated mouse inputs (so native touch is always enabled, e.g. Wayland).
+
 ``--input-ar-delay``
     Delay in milliseconds before we start to autorepeat a key (default: 200).
     Set it to 0 to disable.

options/options.c

@@ -169,6 +169,7 @@ static const m_option_t mp_vo_opt_list[] = {
     {"keepaspect-window", OPT_BOOL(keepaspect_window)},
     {"hidpi-window-scale", OPT_BOOL(hidpi_window_scale)},
     {"native-fs", OPT_BOOL(native_fs)},
+    {"native-touch", OPT_BOOL(native_touch)},
     {"show-in-taskbar", OPT_BOOL(show_in_taskbar)},
     {"display-fps-override", OPT_DOUBLE(display_fps_override),
         M_RANGE(0, DBL_MAX)},

options/options.h

@@ -56,6 +56,7 @@ typedef struct mp_vo_opts {
     bool keepaspect_window;
     bool hidpi_window_scale;
     bool native_fs;
+    bool native_touch;
     bool show_in_taskbar;
 
     int64_t WinID;