libweston/compositor: Do not map subsurfaces without buffer

We can end in `subsurface_committed()` in different scenarios
without the surface having an attached buffer. While setting
the mapped state to `true` in that case doesn't matter for
that (sub)surface itself, it triggers its own child subsurfaces
to get mapped when they shouldn't.

Closes https://gitlab.freedesktop.org/wayland/weston/-/issues/426

Signed-off-by: Robert Mader <robert.mader@collabora.com>
(cherry picked from commit 8b04534c76)

libweston/compositor.c

@@ -4364,7 +4364,7 @@ subsurface_committed(struct weston_surface *surface, int32_t dx, int32_t dy)
 	 */
 
 	if (!weston_surface_is_mapped(surface)) {
-		surface->is_mapped = true;
+		surface->is_mapped = surface->buffer_ref.buffer != NULL;
 
 		/* Cannot call weston_view_update_transform(),
 		 * because that would call it also for the parent surface,