Fix visual glitches in WebGL font rendering

[davepagurek]

Resolves #5852

Changes

• Slightly expands the bounding box of glyphs when picking which grid rows/columns they overlap with to fix the slight glitches you see when they're very close to grid edges
• Slightly expands the bounding box of rendered glyphs on canvas to allow for better antialiasing on edges

Screenshots of the change

Here's a gif that flips between before/after images so the difference stands out:

Here's a visualization of the bounding box sizing changes going on under the hood to fix edge antialiasing:

You can test out the changes on a few different fonts and sizes here: https://editor.p5js.org/davepagurek/sketches/l1ulBxXnK

PR Checklist

• npm run lint passes