This is WIP. There's still some atomicity errors in here.

Public issue https://github.com/mackron/miniaudio/issues/318

A device job thread is used for processing jobs from a separate thread
for cases where doing a certain operation needs to be done from outside
a callback.

This is in preparation for generalizing the job system.

This will now pick the closest miniaudio-supported format to the source
file's internal format rather than always defaulting to f32.

The issue here is that MA_AT_END is getting returned when nothing has
been read. The rules are that MA_AT_END should only ever be returned
when nothing is read.

This is important for fixed sized processing.