Add support for QUADS and QUAD_STRIP in WebGL mode

[davepagurek]

Resolves #4401

Changes:
This handles the QUADS and QUAD_STRIP shape modes in the WebGL renderer.

WebGL 1 doesn't support those two modes, so they have to be drawn using other modes. I've converted QUADS to TRIANGLES and QUAD_STRIP to TRIANGLE_STRIP.

Turning QUADS into triangles requires some vertices to be duplicated:

• I've added code into vertex() that, on the fourth vertex of each quad, duplicates two vertices so that, when the final vertex is added, two complete triangles have been added to the buffer
• It also duplicates normal, uv, and fill color data so that nothing gets misaligned
• This is a tad complicated, so I've added a unit test to make sure the triangle vertices are added correctly
• When computing edges, only the external lines of the quad are used

Turning QUAD_STRIP into TRIANGLE_STRIP is simpler:

• For fills, nothing needs to be changed! It's already in the right format
• For edges, only lines on the outer edge of quads are used

Screenshots of the change:

I've added a manual test comparing, from left to right, TRIANGLE_STRIP, QUADS, and QUAD_STRIP modes for the same vertex data. I've added random fill colors to each vertex to make sure they get duplicated correctly.

PR Checklist

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

[stalgiag]

What a beautiful, considered, thorough, and speedy PR!

Minor comments inline.

[stalgiag]

Looks great to me!