Fix lockup in creator.

My previous fix to force expose when the render thread is inactive
was not enough. We now lock down access to 'QSGRenderThread::window'
so that it will always be set when the thread is in the "exposed"
state and 0 when the thread is in the "obscured" state.

This introduces another sync point in handleObscurity to protect
the writing of window in the render thread.

Task-number: QTCREATORBUG-10793
Change-Id: I1e1153189b3a3562705892b42625f88ef6329188
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>