player: add window-id property

currently only supported on x11.

one practical use-case of this is wanting to embed something (such as
dmenu) into the mpv window to use as a menu/selection. there might be
other use-cases as well (e.g doing some shenanigans with `xdotool` or
whatnot).

it's currently possible to:

* listen for 'current-window-scale' change (to check if the
  window has been created or not)
* call an external tool like `xdo` or `xdotool` and grab the xid
  from mpv's pid.

however it adds unnecessary dependency on external tools when mpv is
fully capable of easily providing this information.

closes: #10918

DOCS/man/input.rst

@@ -2652,6 +2652,10 @@ Property list
     Any of these properties may be unavailable or set to dummy values if the
     VO window is not created or visible.
 
+``window-id``
+    Read-only - mpv's window id. May not always be available, i.e due to window
+    not being opened yet or not being supported by the VO.
+
 ``mouse-pos``
     Read-only - last known mouse position, normalizd to OSD dimensions.
 

player/command.c

@@ -1344,6 +1344,17 @@ static int mp_property_idle(void *ctx, struct m_property *prop,
     return m_property_flag_ro(action, arg, mpctx->stop_play == PT_STOP);
 }
 
+static int mp_property_window_id(void *ctx, struct m_property *prop,
+                                 int action, void *arg)
+{
+    MPContext *mpctx = ctx;
+    struct vo *vo = mpctx->video_out;
+    int64_t wid;
+    if (!vo || vo_control(vo, VOCTRL_GET_WINDOW_ID, &wid) <= 0)
+        return M_PROPERTY_UNAVAILABLE;
+    return m_property_int64_ro(action, arg, wid);
+}
+
 static int mp_property_eof_reached(void *ctx, struct m_property *prop,
                                    int action, void *arg)
 {
@@ -3616,6 +3627,7 @@ static const struct m_property mp_properties_base[] = {
     {"seekable", mp_property_seekable},
     {"partially-seekable", mp_property_partially_seekable},
     {"idle-active", mp_property_idle},
+    {"window-id", mp_property_window_id},
 
     {"chapter-list", mp_property_list_chapters},
     {"track-list", property_list_tracks},

video/out/vo.h

@@ -126,6 +126,7 @@ enum mp_voctrl {
     VOCTRL_GET_DISPLAY_FPS,             // double*
     VOCTRL_GET_HIDPI_SCALE,             // double*
     VOCTRL_GET_DISPLAY_RES,             // int[2]
+    VOCTRL_GET_WINDOW_ID,               // int64_t*
 
     /* private to vo_gpu and vo_gpu_next */
     VOCTRL_EXTERNAL_RESIZE,

video/out/x11_common.c

@@ -2118,6 +2118,12 @@ int vo_x11_control(struct vo *vo, int *events, int request, void *arg)
         ((int *)arg)[1] = selected_disp->rc.y1 - selected_disp->rc.y0;
         return VO_TRUE;
     }
+    case VOCTRL_GET_WINDOW_ID: {
+        if (!x11->window)
+            return VO_NOTAVAIL;
+        *(int64_t *)arg = x11->window;
+        return VO_TRUE;
+    }
     case VOCTRL_GET_HIDPI_SCALE:
         *(double *)arg = x11->dpi_scale;
         return VO_TRUE;