Halt/stop channel playback

If channel == -1, halt all channels.

Property definitions

sdl2 $ Mix :: halt_channel
	# Halt/stop `channel` playback
	#
	# If `channel == -1`, halt all channels.
	fun halt_channel(channel: Int) `{
		Mix_HaltChannel(channel);
	`}
lib/sdl2/mixer.nit:112,2--117,3