libretro_volume_fixes

These per-game core options will re-balance the volume in order to have less loud sound effects and enjoing the BGM more.

installation:

1. make sure you are using a build of the libretro cores with my PRs for per-channel volume options;
2. download this archive and extract in your Retroarch /config dir (make sure the matching cores .opt files are there).
3. the volume fixes will autoload with the supported games and cores.