The pool adjusts to the amount of required threads.
The implementation is as simple as possible, but should be enough
for future extensions.
This brings down decoder thread creation/destruction even more.
Also fix OpenALSystem::release() to close channels properly on shutdown,
otherwise it would deadlock with the pool, because it waits until all
threads have died off, which did not necessarily happen.
Apparently this was half implemented but not fully finished.
Mono samples work fine, for stereo, L/R panning does not work (due to OpenAL's limitation)
Also fixed: entity sounds should not play if a cutscene is currently skipped.
