This catches the case when backends do not return MA_AT_END when no
data is read.

This updates ma_encoder_write_pcm_frames() to be consistent with
reading, where a result code is returned, and the number of frames
written is returned via an output parameter.

With this change, an error is now returned when the requested frame
count is zero. Two reasons for this:

  1) It usually means there's an error in the higher level logic if
     something is trying to read zero frames.

  2) When no frames are read, MA_AT_END should be returned. However, if
     the input frame count is also zero, it creates ambiguity as to
     whether or not the data source is truly at the end.

This commit also decouples the job queue from the resource manager
which will make it easier to use in other areas if it ever comes up.

Public issue https://github.com/mackron/miniaudio/issues/341

  * If starting the backend fails, don't attempt to enter the data loop
  * If stopping the backend fails, don't fire the stop callback.

Public issue https://github.com/mackron/miniaudio/issues/344