Finds the first available channel in a group of channels.
int Mix_GroupAvailable(int tag);
an arbitrary value, assigned to channels, to search for.
Returns first available channel, or -1 if none are available.
A tag is an arbitary number that can be assigned to several mixer channels, to form groups of channels.
This function searches all channels with a specified tag, and returns the channel number of the first one it finds that is currently unused.
If no channels with the specified tag are unused, this function returns -1.
This function is available since SDL_mixer 2.0.0.