Make sure setAttributes updates lingering references to the old canvas

[davepagurek]

Resolves #5817

Changes

Previously, when setAttributes creates a new canvas, it wouldn't update the canvas property of the renderer, leaving a reference to the old canvas. Now, when it's set correctly, sketches that try to read back the main canvas via a reference to a renderer will see the contents of the canvas instead of a blank canvas.

Screenshots of the change

Before:

• Open https://editor.p5js.org/hlp/sketches/ncJGM9DNj
• Add setAttributes({ alpha: true }) to setup

After:

• Open https://editor.p5js.org/davepagurek/sketches/794nSzyvE

PR Checklist

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