Script idea... Preview MultiMatte passes in the viewport

Hi folks,

Maybe it's just the disorganised projects that keep getting dropped on my poor head, but this'd be super handy for dealing with maybe dozens of MultiMatte Render Elements in a scene and would give near instant feedback to check your passes instead of needing a render.

Something like:

- Build a list of Render Elements in the scene
- Back up a Scene State of the current materials setup
- Build a list of MultiMattes, select one from a dropdown
- Check if it's using MatIDs or ObjIDs
- Apply an unshaded black material to everything
- Select R channel objects by ID, apply a red material
- Same for G and B...
- Viewport now previews what the matte output will be for that pass
- Modify IDs as necessary, update viewport to see results
- Restore original Scene State to return to where we started

Probably a much more elegant way of doing this, but that's about the extent my pseudocode. I've had a go at bodging something together with bits of other scripts, but pretty quickly ran into a wall. Ooft.

Cheers!