Fix MTL color loading when vertices are shared across different faces

[davepagurek]

Changes:

A user on Discord discovered that their obj + mtl combo was not rendering correctly:

Zipped obj + mtl here

It turns out this can happen when vertices are reused between faces. Previously, when a vertex is used by a face for the first time, it adds the vertices to the p5.Geometry only if it has not been used in that color before. Then, when the face is added, it adds colors once per vertex, regardless of whether it is new or not. This means we might add a color to the color buffer a second time for a vertex that already exists in the buffer, pushing all subsequent colors out of sync.

Now, colors are added at the same time as other vertex properties. This all-or-nothing structure will hopefully help prevent some properties from going out of sync.

PR Checklist

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