Handle premultiplied alpha in filter(OPAQUE)

[davepagurek]

Resolves #6528

Changes

• Handles the fact that filter input has its alpha channel premultiplied. It needs to be unmultiplied to not result in darker colors before changing the alpha.

Screenshots of the change

https://editor.p5js.org/davepagurek/sketches/OXcL-h7ui

This sketch is now full red instead of dark red.

Before:

After:

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated

[inaridarkfox4231]

I feel like a similar problem is probably occurring with POSTERIZE and THRESHOLD(though I'm not sure how important handling of low opacity is...)

[davepagurek]

good catch @inaridarkfox4231! I've updated those shaders, live here: https://editor.p5js.org/davepagurek/sketches/HBU7vSmuk