xwayland: Call glamor_block_handler from xwl_screen_post_damage

In between the two phases introduced by the previous change. This makes
sure all pending drawing to the new buffers is flushed before they're
committed to the Wayland server.
(cherry picked from commit a542224e)

hw/xwayland/xwayland.c

@@ -43,6 +43,10 @@
 _X_EXPORT Bool noXFree86VidModeExtension;
 #endif
 
+#ifdef XWL_HAS_GLAMOR
+#include <glamor.h>
+#endif
+
 void
 ddxGiveUp(enum ExitCode error)
 {
@@ -850,6 +854,13 @@ xwl_screen_post_damage(struct xwl_screen *xwl_screen)
     if (xorg_list_is_empty(&commit_window_list))
         return;
 
+#ifdef XWL_HAS_GLAMOR
+    if (xwl_screen->glamor &&
+        xwl_screen->egl_backend == &xwl_screen->gbm_backend) {
+        glamor_block_handler(xwl_screen->screen);
+    }
+#endif
+
     xorg_list_for_each_entry_safe(xwl_window, next_xwl_window,
                                   &commit_window_list, link_damage) {
         wl_surface_commit(xwl_window->surface);