Separate channel export: Triangle contains other channels at a low volume

[AnotherGoat]

When exporting project audio as WAV/MP3/OGG in separate files for each channel and listening to the exported Triangle channel, the Noise and DPCM channels can be heard at a lower volume.

This happens both when doing it using the song export UI and ticking "Separate channel files" and when using the CLI like this:

famistudio test_project.fms wav-export "song.wav" -wav-export-separate-channels

Minimal Reproduction Project

[BleuBleu]

On the NES, the triangle/noise/dmc all influence each other in a subtle way. So when exporting separate channels, there will be some bleed through. More info here : https://www.nesdev.org/wiki/APU_Mixer

There used to be a an documented feature to change this behavior a while ago, but it broke since no one was using it. Ill keep this issue open to remind me of fixing it one day.

-Mat