Support beginShape(TESS) for faces that don't face the camera

[davepagurek]

Resolves #5913

Previously, all vertices sent to libtess were assumed to be on the XY plane. Libtess can fit a plane to the vertices and use its normal for tesselation, if we tell it the plane's normal is (0,0,0). Presumably this is a little bit slower than not having to do that step, so rather than doing that all the time, I first check if all the vertices have the same z value (likely the most common use case) and tell libtess to use (0,0,1) as the normal if so. Otherwise, it calculates its own normal to use.

Screenshots of the change:

Before: (faces perpendicular to the initial camera turn into holes)

After:

Live: https://editor.p5js.org/davepagurek/sketches/X8o0LFCGd

PR Checklist

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