Allow PSG User Envelope to be applied per Channel and Program

[vitaflo]

I've been playing with PSG Envelopes on MDMI. I've noticed any user envelope basically overrides all the program envelopes and also applies to all PSG channels.

Since there were only two simple envelopes defined in the code I manually added a few new envelopes to the array in envelopes.c and recompiled the ROM. This lets me apply a specific envelope to a specific PSG channel.

What would be great would be a way to apply a user envelope to one of the programs that I can then use (or not) on any PSG channel, similar to how I think the FM presets work. In essence let me overwrite one of the envelopes in ENVELOPES[MIDI_PROGRAMS] and then use that like any other envelope in that array.

I assume this means changing how the Sysex call works since it would need more info on where to put the envelope.

[rhargreaves]

Ah yeah, this is something which I was intending to get to at some point. The user preset SysEx commands have been designed to be extended to support more than one type (currently type has to be 0 = FM, but 1 was going to be PSG!)

The existing SysEx call to load a PSG envelope is as you say quite basic and really just a sticking plaster before proper preset support arrived (and I prioritised FM support over PSG).

I might be able to take a look at this over the winter break, but I have other work on at present which has my attention.

[vitaflo]

Ahh yes now I see the stub for STORE_PROGRAM_TYPE_PSG in the repo. I'd try and help do a PR for this but my C skills are lacking.

One nice feature if/when you get this working is to also add support for it on MDMI CLI, and allow us to use EEF format for the envelopes (and load from .eef files). That would make loading (and sharing) of envelopes really easy.