[headless] Don't send updates when pending resize
[headless] Don't send updates when pending resize

There was a race condition where if an update happened after a resize
request was sent before the surface was actually changed, update signals
with incorrect surface width/height could be sent.

Make sure we don't send updates when a resize is pending. Fixes
crashes/corrupt updates in ClutterMozEmbed.

clutter-project/clutter-mozembed,clutter-project/clutter-mozembed,clutter-project/clutter-mozembed