Use event queue to decouple audio playback from the battle system. Switch
back to using begin and end attack within the entity update loop for
conciseness.
On intermittent stop of OpenAL playback deleting the buffers without
unqueuing the current buffers is an invalid operation, causing invalid
audio states on alGetSource().
Audio can be played with 1 call, audio_beginVorbisStream which
encapsulates the initialisation of the renderer and release on finish.
OpenAL is quoted to have a limitation of around 32 sources. Audio
renderers hence acquire a source slot on playback and immediately release
on finish. This ensures that sources are held optimally and minimises the
possibility of saturating the available sources.
Renderers hold an index to the source list, which the index reveals the
source ID for their use.
