clutter: Hook up ClutterStageView render-to-texture

"Blit" the result on the framebuffer after each view is painted.
This of course only applies if there is an offscreen buffer to
perform any blitting. Otherwise the onscreen framebuffer is rendered
to directly.

https://bugzilla.gnome.org/show_bug.cgi?id=745079

clutter/clutter/clutter-stage.c

@@ -676,6 +676,12 @@ clutter_stage_do_paint_view (ClutterStage                *stage,
   _clutter_stage_paint_volume_stack_free_all (stage);
   _clutter_stage_update_active_framebuffer (stage, framebuffer);
   clutter_actor_paint (CLUTTER_ACTOR (stage));
+
+  if (clutter_stage_view_get_onscreen (view) !=
+      clutter_stage_view_get_framebuffer (view))
+    {
+      clutter_stage_view_blit_offscreen (view, clip);
+    }
 }
 
 /* This provides a common point of entry for painting the scenegraph