Change function arguments for MIDI channel to zero-based

[SpotlightKid]

Currently the channel argument for the sfizz_send_* functions uses 1-based counting of MIDI channels.

I suggest changing this to zero-based counting, i.e. 0-15, since this is how the channel is represented in the MIDI status byte and how most software stores and passes it internally.

IMO, 1-based counting should only be used when presenting a MIDI channel number to the user.

[paulfd]

Agreed!

[paulfd]

Should be ok on develop as of c6a9ba8