* stage1: attempt to make Easy easier; misc tweaks
* stage1: buff health of boss nons
* stage1: use new enemy spawners
* stage1: Normal balance; some Easy tweaks
* stage1: tweak Hard version of Perfect Freeze
* stage1: simplify a common_charge
This replaces SDL_mixer with an internal streaming and mixing system,
relying only on basic SDL audio support. It's also a partial refactor of
the audio API, most notably BGM-related. The BGM metadata resource type
is gone, as well as the `.bgm` files. The metadata is now stored inside
the `.opus` files directly, using standard Opus tags.
