The value of priority was read without the mutex locked, from within
the thread.
Had to extract a QThreadPrivate::setPriority method so that it can be called
with the mutex already locked. So if the main thread calls setPriority
while the thread is starting, it will be either be before or after the
"re-set priority" code at thread startup, but at least not in the middle of it.
Change-Id: I7a054f68623f61482c749274da66f3b2dcd8bcee
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
d4adee7851