clutter-stage: Don't always set the projection and viewport matrix

The stage has a dirty flag to record whenever the viewport and
projection matrices need to be flushed. However after flushing these
the flags were never cleared so it would always redundantly update the
state.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2480

clutter/clutter-stage.c

@@ -2690,10 +2690,16 @@ _clutter_stage_maybe_setup_viewport (ClutterStage *stage)
                          priv->viewport[2],
                          priv->viewport[3]);
 
+
+      priv->dirty_viewport = FALSE;
     }
 
   if (priv->dirty_projection)
-    cogl_set_projection_matrix (&priv->projection);
+    {
+      cogl_set_projection_matrix (&priv->projection);
+
+      priv->dirty_projection = FALSE;
+    }
 }
 
 /**