* Implement the notion of a virtual file system (VFS) which is used
    by the resource manager for loading sound files. The idea is that
    the application can implement these to support loading from custom
    packages, archives, etc.
  * Add a helper API for decoding a file from a VFS and a file name.
  * Add some symbols representing allocation types. These are not
    currently used, but I've added them in preparation for changes to
    the allocation callbacks. The idea is that an allocation type will
    be passed to the callbacks to give the allocator better intel as to
    what it's allocating which will give it a chance to optimize.
  * Add some placeholders for flags for controlling how to load a data
    source. Currently only MA_DATA_SOURCE_FLAG_DECODE is implemented
    which is used to indicate to the resource manager that it should
    store the decoded contents of the sound file in memory rather than
    the raw (encoded) file data.
  * Support has been added to the resource manager to load audio data
    into memory rather than naively reading straight from disk. This
    eliminates file IO from the audio thread, but comes at the expense
    of extra memory usage. Support for streaming is not implemented as
    of this commit. Early (largely untested) work has been implemented
    to avoid loading sound files multiple times. This is a simple ref
    count system for now, with hashed files paths being used for the
    key into a binary search tree. The BST is not fully tested and
    likely has bugs which will be ironed out in future commits.
  * Support has been added for configuring the stereo pan effect. Most
    audio engines use a simple balancing technique to implement the
    pan effect, but a true pan should "move" one side to the other
    rather than just simply making one side quieter. With this commit,
    the ma_panner effect can support both modes. The default mode will
    be set to ma_pan_mode_balance which is just a simple balancing and
    is consistent with most other audio engines. A true pan can be used
    by setting the mode to ma_pan_mode_pan.

This was causing mmaped data sources to not loop if they coincidentally
reached the end at the same time as the final output frame was written.

This changes makes ma_copy_pcm_frames() a no-op when the input and
output buffers are both set to the same pointer. This is useful for
some in-place no-ops for effects and filters.

This may be changed back to a first order LPF later on.

This is required so we can do different spatialization logic depending
on the channel count of the audio source.

There is a bug that's causing glitches which appears to be something to
do with the second order LPF.

This was affecting the stability of the internal low pass filters.

This bug is happening due to the internal timer not being adjusted to
match the new sample rate.

This currently only supports f32, but support for more formats will be
coming soon.

  * Initial work on infrastructure for spatialization, panning and
    pitch shifting.
  * Add ma_engine_sound_set_pitch()

Spatialization and panning is not yet implemented, but pitch shifting
should now be working.

  * The engine will now auto-start by default. This can be changed in
    the config by setting `noAutoStart` to true.

  * Initial implementation of ma_engine_play_sound() which can be used
    for fire-and-forget playback of sounds.

  * Add ma_engine_sound_at_end() for querying whether or not a sound
    has reached the end. The at-end flag is set atomically and
    locklessly in the mixing thread.

This change makes it so MA_AT_END is returned when a non-looping data
source reaches the end. This is how the caller can know to do clean up
or whatnot.

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()