Fix corrupted textures when rendering too many WebGL characters

[davepagurek]

Resolves #7787

Changes:

• Caches WebGL font data based on the individual data textures rather than per glyph (since glyphs share some of the data textures)
• Removes font IDs since we no longer need to create a custom cache key -- the cache can be based on unique ImageData object references now
• Adds a visual test to prevent regressions

Screenshots of the change:

Before:

After:

Live: https://editor.p5js.org/davepagurek/sketches/HOCMo0I8F

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated