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.

This is the new default. The size of the buffer will be based on the
periodSizeInFrames and periodSizeInMilliseconds config variable. To go
back to the previous system, where the data callback could specify any
number of frames, set the noFixedSizedCallback config variable to true.

This flag is used to tell miniaudio that it should assume the output of
the node is silence. This is useful for special nodes that want to
output to a file and don't want the output of the node to contribute to
the final mix.

Public issue https://github.com/mackron/miniaudio/issues/388