Set the music volume out of mix.max_volume and return the previous value

Use volume = -1 to only read the previous value.

Property definitions

sdl2 $ Mix :: volume_music
	# Set the music volume out of `mix.max_volume` and return the previous value
	#
	# Use `volume = -1` to only read the previous value.
	fun volume_music(volume: Int): Int `{
		return Mix_VolumeMusic(volume);
	`}
lib/sdl2/mixer.nit:184,2--189,3