Matplotlib spheres don't clip their child light disks

[bdice]

Original report by Matthew Spellings (Bitbucket: mspells, GitHub: klarh).

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This could potentially be a problem if the light direction veers too far away from (0, 0, -1). Fighting with matplotlib to get it to actually clip the disks was becoming burdensome, so I left it as a TODO.

This probably doesn't matter too much in the grand scheme of things, since it's so hard to see what's going on in any nontrivial matplotlib-rasterized scene at the moment.