Some few changes to libopenmpt branch

All: A limiter was applied so the music don't get disorted with certain formats. Add missing MUS_FLAC case to the switch statement.

libopenmpt: Only call openmpt_module_set_repeat_count(mod, -1) if looping is set. Move openmpt_module_set_render_param out of the mix hook and into I_PlaySong and modfilter cvar, so it can apply on song playback and cvar value change.