glx: Make sure to glFlush if blitting to frontbuffer.

Unlike glXSwapBuffers, glXCopySubBuffer and glBlitFramebuffer don't
issue an implicit glFlush() so we have to flush ourselves if we want the
request to complete in finite amount of time since otherwise the driver
can batch the command indefinitely.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2551

clutter/glx/clutter-stage-glx.c

@@ -577,6 +577,13 @@ clutter_stage_glx_redraw (ClutterStageWindow *stage_window)
                                             copy_area.width,
                                             copy_area.height);
       CLUTTER_TIMER_STOP (_clutter_uprof_context, blit_sub_buffer_timer);
+
+      /* NB: unlike glXSwapBuffers, glXCopySubBuffer and
+       * glBlitFramebuffer don't issue an implicit glFlush() so we
+       * have to flush ourselves if we want the request to complete in
+       * finite amount of time since otherwise the driver can batch
+       * the command indefinitely. */
+      glFlush ();
     }
   else
     {