Add edge highlighting

[ianmackenzie]

Various possible approaches:

• Draw lines, apply polygon offset to faces (simplest but mediocre quality/performance)
• Fragment shader based approaches (relies on including barycentric coordinates or similar in vertex data, requires support for GLSL standard derivatives)
• Post processing of depth map data (requires support for multipass rendering)

[ianmackenzie]

Perhaps use lambertian / fwidth(lambertian) for toon shading, as suggested by https://twitter.com/rickyreusser/status/1023852556060057600 (would require standard derivative support in the webgl package)