* ma_data_source_get_cursor_in_pcm_frames()
  * ma_data_source_get_length_in_pcm_frames()

When the data source has no notion of a cursor or length, these return
MA_NOT_IMPLEMENTED to let the caller know about it. This is returned
when a custom data source leaves these functions unimplemented.

ma_decoder, ma_audio_buffer, ma_waveform and ma_noise have all been
updated to support these new functions.