clutter/cogl: 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 and this step is not necessary.

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

clutter/clutter/cogl/clutter-stage-cogl.c

@@ -419,6 +419,12 @@ paint_stage (ClutterStageCogl            *stage_cogl,
 
   _clutter_stage_maybe_setup_viewport (stage, view);
   _clutter_stage_paint_view (stage, view, clip);
+
+  if (clutter_stage_view_get_onscreen (view) !=
+      clutter_stage_view_get_framebuffer (view))
+    {
+      clutter_stage_view_blit_offscreen (view, clip);
+    }
 }
 
 static void