audio: Add al_get_audio_stream_played_samples.

Discussion was here https://www.allegro.cc/forums/thread/613415
--pw
audio: Fix deadlock in Pulseaudio driver.

This arises because when, e.g. stopping the voice, the main thread holds
both the voice->mutex and the pv->status_mutex while waiting for the
background thread to change status to PV_IDLE.  Meanwhile the background
thread enters _al_voice_update which also tries to acquire voice->mutex.
This was not as obvious as described because both calls to acquire
voice->mutex occur outside of this file.

Reported by Bluebird.
audio: Try to show at call site where voice mutex will be acquired by callee.