Fix rounding of values close to 0 when computing smooth normals

[davepagurek]

Resolves #6590

Changes

I've updated the function to compute if vertices are the same to use rounding rather than toFixed.

Let's say you have two vertices, one at x=-1e-8 and one at x=1e-8. Previouisly, using toFixed, these values become -0.000 and 0.000, which are different strings, so they don't end up equal. After this change, both end up 0.

Screenshots of the change

Updated version of the sketch in the issue:
https://editor.p5js.org/davepagurek/sketches/WJ4DtuCcV

PR Checklist

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

[davepagurek]

@capGoblin does this look good to you?

[capGoblin]

alright, makes sense, Let's merge it!