Add faux transparency

[keenancrane]

There are a lot of tricks for rendering transparency; this one is particularly friendly to the rasterization pipeline and could work well in Polyscope (also because it nicely highlights contours):

https://observablehq.com/@rreusser/faking-transparency-for-3d-surfaces

“Cartoon edges” might be replaced with simple edge based silhouettes (i.e., compare the sign of the dot product of the normal with the eye for the 2+ triangles touching an edge), since the former works well only for well-tessellated surfaces.