Sfall-like sound system (#429)

* Sfall-like sound system

(I would like a better name for this system, but the opcodes themselves are called `play|stop_sfall_sound` and I can't think of a better name)

This is a sound management system very similar to Sfall's.  With a few differences:
* No support for new sound formats yet.  Support for at least ogg+wav will come next
* Does support reading from .dat files, hence playing any sound effect in the base game
* Support mode=3 for speech volume.  This exists in Sfall but is ignored when called from the opcode.

Also, change GaplessMusic to use this system to play the "wind2" loading sound.  This restores the "vanilla" feel map transitions, so I think it's reasonable to make GaplessMusic=1 the default.  I can't imagine someone wanting to change it to 0 unless they are ultra purists

third_party/stb_vorbis/LICENSE

@@ -0,0 +1,37 @@
+This software is available under 2 licenses -- choose whichever you prefer.
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