qt: do not repeatedly check the current thread during video surface synchronization

We can have this assumption to not check for the thread each time it is called. This can be expensive, as the method is called at v-sync rate.

We can do this because VideoSurfaceProvider::supportsThreadedSurfaceUpdates() is already probed once and assumed to be constant.

For safety, I have added assertions for the debug build to check if this assumption does not hold.

Request review @chub.