Miniaudio and Soundfont-related fixes/updates

[dashodanger]

This PR rolls up the following:

• Removed unneeded support for DirectSound in Miniaudio
• Fixed Miniaudio not respecting DDF per-sound volume levels
• Fixed looping sound effects not terminating when they should
• Fixed Player/UI category sounds being attenuated to the environment
• Added Z-component (in Miniaudio this is actually the Y component) to listener direction and velocity vectors
• Converted existing SF2 fonts to SF3
• Moved default soundfont to edge_defs and renamed to SNDFONT.sf3; lumps named "SNDFONT" from other loaded files can replace this, as well as pack files named SNDFONT.sf2 or SNDFONT.sf3
• Removed path component of midi_soundfont CVAR; when switching soundfonts, the user/home directory will be checked, and then (if not running in portable mode) the program directory