* ma_audio_buffer_get_available_frames()
  * ma_decoder_get_available_frames()

  * The data buffers and data streams are now first class data sources.
  * The ma_resource_manager_data_source object is now just a simple
    wrapper around ma_resource_manager_data_buffer and
    ma_resource_manager_data_stream.
  * Unnecessary pResourceManager parameters have been removed.
  * The part of the data buffer that's added to the BST has been split
    out from the main data buffer object so that the main object can be
    owned by the caller.
  * Add ma_resource_manager_data_source_get_available_frames() which is
    used to retrieve the number of frames that can be read at the time
    of calling. This is useful in asynchronous scenarios.

  * A crash when the pFramesRead parameter is null
  * Looping not working due to MA_AT_END now being returned.

Public issue https://github.com/dr-soft/miniaudio/issues/185

Public issue https://github.com/dr-soft/miniaudio/issues/184

This should fix the case where multiple context's are initialized and
uninitialized at the same time across multiple threads.

Public issues:

  * https://github.com/dr-soft/miniaudio/issues/163
  * https://github.com/dr-soft/miniaudio/issues/183

This commit changes synchronous decoding so that the calling thread is
the one which performs the decoding. Previously, decoding was done on
the job threads which was then waited on by an event on the calling
thread. The rationale for this design was to keep decoding on a single
code path, however this creates a problem for programs that would
prefer not to have any asynchronous job threads. In this case, these
synchronously decoded sounds would never get decoded because there
would not be any threads available to actually perform the decoding.

This commit enables the resource manager to be able to be used without
a job thread so long as asynchronous decoding and streaming are not
used. This scenario could be useful for programs that want to pre-load
all of their sounds at load time and save some system resources by not
incurring the overhead of an additional unnecessary thread.