Avoid context-switches when not switching thread contexts

When UI and GPU threads are the same we not need to do async scheduling
or calls between them, and doing so comes at a much higher cost than
in a multithreaded environment.

Change-Id: Icddee1a78d87ce08362882da5740471dfef1224a
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>