vo_wayland: define opaque region

This allows compositor to optimize rendering, as it will know mpv is not
transparent.

video/out/wayland_common.c

@@ -795,6 +795,15 @@ static void schedule_resize(struct vo_wayland_state *wl,
     wl->window.events |= VO_EVENT_RESIZE;
     wl->vo->dwidth = width;
     wl->vo->dheight = height;
+
+    struct wl_region *region = wl_compositor_create_region(wl->display.compositor);
+
+    if (region) {
+        wl_region_add(region, x, y, width, height);
+        wl_surface_set_opaque_region(wl->window.video_surface, region);
+        wl_surface_commit(wl->window.video_surface);
+        wl_region_destroy(region);
+    }
 }
 
 static void frame_callback(void *data,