Updated APIs:
  * ma_data_source_read_pcm_frames()
  * ma_data_source_seek_pcm_frames()

This commit also explicitly defines ma_data_source_read_pcm_frames() as
supporting an output buffer of NULL in which case a forward seek should
be performed.

This will cause a forward seek to occur instead of a read when calling
ma_decoder_read_pcm_frames() with the output buffer set to NULL.

  * Add support for volume control to ma_mixer_mix_*().
  * Add support for specifying an effect to apply before mixing.
  * Add optimized pipeline for memory mappable data sources.
  * Remove some unnecessary functions:
    - ma_mixer_mix_pcm_frames_ex()
    - ma_mixer_mix_callbacks()
    - ma_mixer_mix_decoder()
    - ma_mixer_mix_audio_buffer()
    - ma_mixer_mix_waveform()
    - ma_mixer_mix_noise()
    - ma_mixer_mix_rb_ex()

The ma_audio_buffer object is used for storing raw audio data in memory
and reading from it like any other data source. It supports flexible
memory management, reading, seeking, memory mapping and looping.

See documentation under "Audio Buffers" for a detailed description.