Add support for the fontsound loop mode
Accumulate with default modulator amounts
Fix copy-paste errors in error messages
Add volume envelope properties
Add a few more pitch properties
Add modulation envelope properties
Add filter properties
Add tuning properties
Add pan and attenuation properties
Check for negative soundfont indices getting passed to the loader
Add chorus and reverb send properties
Add the last remaining "real-time" properties
Lock the device while selecting soundfonts
Update soundfont selection functions

A filename is no longer specified, and alMidiSoundfontsSOFT is renamed to
alMidiSoundfontvSOFT.
Remove explicit locking from alMidiSoundfontvSOFT.

The MIDI backend can lock as it needs.
Don't destroy the fluid_sfont_t if any of its samples are still playing

FluidSynth apparently doesn't check for this itself before calling the free
callback.
Remove some unused MIDI backend methods