SDL3: update volume to a float

[Starbuck5]

In SDL2, SDL_mixer uses https://wiki.libsdl.org/SDL2/SDL_MixAudioFormat, which takes a volume from 0-128.

In SDL3, SDL_mixer uses https://wiki.libsdl.org/SDL3/SDL_MixAudio, which now takes a float.

According to SDL's README-MIGRATION:

The following symbols have been removed:
SDL_MIX_MAXVOLUME - mixer volume is now a float between 0.0 and 1.0

Therefore, I think it would make sense for SDL_mixer to follow SDL's lead here, before the API is frozen for release.

[slouken]

Agreed, thanks for the PR!