Big refactor to the resource manager.

  * 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.