The tracking of whether it is over a window or not is only updated when the
mouse moves.  If a window was created or moved under it, then the state can be
stale.  That caused us to defer hiding the cursor until the mouse was moved.
This happens at the start of games pretty often.

winemac: Don't use the main dispatch queue to implement OnMainThread() for a thread with no event queue.

The main dispatch queue is a serial queue and is a shared resource.  If we
submit a long-running task to it, then no other tasks, including those submitted
by the system frameworks, can run until it completes.