Allow ensuring that a stage viewport is updated

Allow ensuring that a stage viewport is updated

Since we only update the GL viewport when we receive a ConfigureNotify
event on X11, we also need a function to allow other toolkits to tell
a stage that the viewport should be updated.

This commit adds clutter_stage_ensure_viewport(), a function that simply
sets the private SYNC_MATRICES flag on the stage and then queues a
redraw.

This function should be called by libraries integrating Clutter with
other toolkits, like clutter-gtk or clutter-qt.