wayland: block after creating image_desc to ensure it gets set

Previous commit fixed the problem that `set_color_management` would be
called on every frame, however it didn't fix the problem that due to a
lack of blocking, we'd end up doing events in the following order:

  -> wl_surface#9.commit()
  -> wp_color_management_surface_v1#45.set_image_description(...)
  -> wl_surface#9.commit()
  -> wp_color_management_surface_v1#45.set_image_description(...)

This would mean the image description would always lag behind by 1
commit, and effectively the first frame would _always_ have no image
description set.

Fix this issue by blocking the thread until the compositor responds with
a ready event and we call set_image_description.

Author: llyyr

video/out/wayland_common.c

@@ -3491,6 +3491,10 @@ static void set_color_management(struct vo_wayland_state *wl)
     }
     struct wp_image_description_v1 *image_description = wp_image_description_creator_params_v1_create(image_creator_params);
     wp_image_description_v1_add_listener(image_description, &image_description_listener, wl);
+
+    /* Do a round trip to ensure the image description gets set before
+     * vo->driver->flip_page calls wl_surface_commit on us */
+    wl_display_roundtrip(wl->display);
 #endif
 }