- Re-normalize all BGMs to the same volume
- Merge intro/loop parts into a single file
- For BGMs with intros, specify the loop point in the .bgm file
closes #99
replaced play_sound_cooldown with play_sound_ex. it now has a new
parameter: bool replace. if true, the sound will interrupt the previous
instance of itself being played, if any. if used many times in
succession, this effectively allows one to "restart" a sound instead of
letting many instances overlap. this sounds better in some instances,
and avoids flooding the channels.
this is also implicitly true for play_ui_sound now.
