Add point cloud export

[Carifio24]

This PR adds an implementation of a "point cloud" export. This uses the POINTS rendering mode for glTF/glB, and the UsdGeomPoints mode for USD.

Pros of these modes:

• Produce lightweight files compared to our scatter spheres method
• Very quick to export

Cons:

• These look nice from a distance, but generally not as good close-up, as the points will generally not increase in size (ultimately based on the renderer). This makes the structure hard to see from near/inside the figure
• (glTF only): The spec has recommendations on how to render these, but that's all. There's no option for setting point width
• (USD only) There is a width option for points, but lots of renderers don't seem to support this mode (including an iPhone 13 Pro). The only one I've seen thus far that does is an ASPOSE viewer, where our outputs look fine.