Add Polyphony to 2A03 and VRC6

[xandramax]

Description

Implements #8

Add support for VCV Rack polyphony. Polyphony is implemented naively, without parallel processing of polyphony channels, which realistically may be the only possible method due to PotatoChips' use of external libraries for synthesis.

Witness a chamber orchestra of de-tuned Nintendo Entertainment Systems!

Type of change

Please select all relevant options:

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Tested with MIDI input (anlexmatos/MIDI-Delexander) with MIDI module in both channel-rotation and unison modes. Patch is attached.

Test Configuration

• Operating System: Windows 7
• C++ compiler version: gcc 9.2.0 (MSYS2)

Checklist

• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [X] I have tested that my fix is effective or that my feature works

[xandramax]

The existing code used the word "channel" to refer to the individual oscillators of a given sound chip. This conflicts with VCV Rack's use of the word "channel" to refer to one of up to 16 elements of a polyphonic cable. So, to avoid confusion, I replaced all existing references from "channel" to "oscillator" instead.