OpenAL: don't reset timestamp on buffer underrun.
OpenAL: don't reset timestamp on buffer underrun.

adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,adamlwgriffiths/Pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,seeminglee/pyglet64,niklaskorz/pyglet,niklaskorz/pyglet,niklaskorz/pyglet