[fluid-dev] MIDI mode

[josh]

So I went ahead and added GM On/Off and GS Reset SYSEX handling.   
There are now 2 parameters synth.midi-mode=normal/gm/gs and  
synth.midi-mode-lock=no/yes.  If midi-mode-lock is set to yes then  
SYSEX MIDI mode messages have no effect (useful for forcing FluidSynth  
to a particular mode).  If the current MIDI mode is GM or GS, then  
percussion note-offs are ignored for all except Long Guiro and Long  
Whistle, as per the GM spec.  I tested this with a GS file and it  
seems to work as expected (no more missing drums).

TODO:
1 Reset MIDI mode as part of fluid_synth_system_reset()
2 Reset controllers when GM is turned on (and other values mentioned  
in gmguide2.pdf)
3 Figure out how to synchronize the midi-mode setting with the  
midi_mode variable, when it is being assigned from the synthesis  
context.

For item 3.  I'm thinking of adding value get callback functionality  
to settings, so that a callback can be registered for obtaining the  
current value (so for example the midi-mode string setting could be  
generated from the current value of the enum synth->midi_mode).   
Perhaps some hack can just be implemented for now though, but calling  
settings functions from within synthesis context isn't really a good  
idea (mutex locks, mallocs, etc).

Feedback and testing would be appreciated :)  The "normal" value for  
midi-mode doesn't sound great, but I couldn't think of a better word.

I'm going to do my best to get FluidSynth 1.1.0 released by the end of  
this month (in a nice working/stable state with good API  
documentation).  Testing FluidSynth in various environments and  
reporting success/problems would be really helpful.  There are some  
fairly significant changes with the whole synthesis path and the more  
testing FluidSynth in real word scenarios the better.

Cheers!
Josh