fmodAudioManager DSP fix and setVolume improvement

Please find below my fix for the problem I reported here:

Currently DSP effects are applied to the end of the whole FMOD DSP chain, which means using configureFilters on any one fmodAudioManager changes all audio output, not just that coming out of the target manager.
The fix was to add a “channel group” for each manager and apply the DSP effects to the group. Incidentally this also allows for individual volume control of each manager using setVolume/getVolume which was not possible before.
I did the fix on the 1.5.4 code, but it looks like it hasn’t been modified since.

I plan to do a bit more work in there, maybe add support for multiple listeners (useful for split-screen play).

oops this should have gone in the Compiling or Editing the Panda Source forum.