SCOPE feature request: repurpose trigger input for blanking control when in XY mode

[katef]

Hi! I think the trigger input is unused when in XY mode. Looking at the source:
https://github.com/VCVRack/Fundamental/blob/v2/src/Scope.cpp#L103-L106

// Trigger immediately in Lissajous mode, or if trigger detection is disabled
if (lissajous || !trig) {
	triggered = true;
}

A few projects use XY mode on physical scopes to render graphics. These look best when there's control with a blanking input, to turn off the trace when it's moving between different parts of the image.

My suggestion is to have the unused trigger input act as a gate to control blanking the signal. I'd default to having the trace draw, and an active-high to turn off the trace.

I'm sure you've seen many examples of impressive oscilloscope art that uses this idea.

Here's an example where I'd love to have control over blanking - you can see the unwanted traces between the letters:

Thank you!