Don't lose application redraws

Our usage of DamageReportBoundingBox was causing us to miss some
updates when an area of the screen was drawn twice in rapid
succession. Add an explicit XSync() call to force the server
to flush rendering to the kernel before we draw.

https://bugzilla.gnome.org/show_bug.cgi?id=657071

src/compositor/meta-window-actor.c

@@ -2300,6 +2300,27 @@ meta_window_actor_pre_paint (MetaWindowActor *self)
       meta_error_trap_push (display);
       XDamageSubtract (xdisplay, priv->damage, None, None);
       meta_error_trap_pop (display);
+
+      /* We need to make sure that any X drawing that happens before the
+       * XDamageSubtract() above is visible to subsequent GL rendering;
+       * the only standardized way to do this is EXT_x11_sync_object,
+       * which isn't yet widely available. For now, we count on details
+       * of Xorg and the open source drivers, and hope for the best
+       * otherwise.
+       *
+       * Xorg and open source driver specifics:
+       *
+       * The X server makes sure to flush drawing to the kernel before
+       * sending out damage events, but since we use DamageReportBoundingBox
+       * there may be drawing between the last damage event and the
+       * XDamageSubtract() that needs to be flushed as well.
+       *
+       * Xorg always makes sure that drawing is flushed to the kernel
+       * before writing events or responses to the client, so any round trip
+       * request at this point is sufficient to flush the GLX buffers.
+       */
+      XSync (xdisplay, False);
+
       priv->received_damage = FALSE;
     }