Revert "use cond_mutex to avoid broadcasting condition before it initialized"

This reverts commit ca2a84d5e0bdce309619400fd63558ffd15a093d.
delete queue_mutex, its work is already done by pqData->lock
rework locking scheme for presentation queue. Again

* stop cancelling presentation thread;
* use pqtarget's lock as cond mutex;
* try to wait for worker thread to get to work before exitting pqCreate()