(ThreadSync) Ensure we continue to wait if we're still post-rendering

(ThreadSync) Ensure we continue to wait if we're still post-rendering

[Problem]  If we're post-rendering and UpdateReady is called, we start another render rather than
           waiting for the post-render to finish.
[Solution] Use a while loop in PostRenderWaitForCompletion instead so that even if we're triggered
           by the update-thread, we don't carry on until the post-render is actually complete or
           if we need to replace the surface.

Change-Id: Id285a8ef09bb74d183534d7011c7a40eb45a6b0b