* 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.

This addresses some clipping issues when accumulating.

ma_zero_pcm_frames() will be removed in version 0.11.