Add an exhaust delay to QSGGuiThreadRenderLoop. Some updates
may be done with posted events, and maybeUpdate event competed
with those, leading to partial updates and frames drawn twice.

Change-Id: I532bff692c597eeba5bbd6def89ae68c80fdd69b
Done-with: Mikko Harju <mikko.harju@jollamobile.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>