This adds support for having a sound fade in when it is started and
fade out when it is stopped.

This commit does not yet include support for fading out when the sound
approaches the end - it only fades out when explicitly stopped with
ma_sound_stop().

The fade time is set in milliseconds.

This commit includes a new effect called ma_fader, but it currently
only supports f32 formats. Support for other formats will be added in
the future.