Add support for volume smoothing to sounds.

Smoothing is disabled by default. To enable it, you must use
ma_sound_init_ex() and configure it via the volumeSmoothTimeInPCMFrames
member of ma_sound_config.

This commit also fixes a bug where ma_gainer is not properly applying
smoothing.

CHANGES.md

+v0.11.15 - TBD
+==============
+* Fix a bug in ma_gainer where smoothing isn't applied correctly thus resulting in glitching.
+* Add support for smoothing volume changes of sounds with `ma_sound_set_volume()`.
+
+
 v0.11.14 - 2023-03-29
 =====================
 * Fix some pedantic warnings when compiling with GCC.