Add Capsules3D archetype.

[kpreid]

What

Part of #1361.

Adds the archetype Capsules3D, component Length, and a matching visualizer.

Capsules are defined by their length and radius, and then transformed in the usual fashion to set their orientation. There is also a constructor for joining a pair of endpoints.

Missing parts that should make it into this PR before it leaves draft:

• Needs an example image — I assume I don't have the permissions to upload one.
• Needs C++ or Python _ext and snippets — I will add these before the PR leaves draft.

Further work not included in this PR, that will add complications:

• There is no wireframe mesh support (expressed as having no FillMode component). This is because it was recommended that I use re_math’s capsules, but re_math has no support for generating wireframes. In the long run, we will want to support tapered capsules and cylinders and a new or substantially revised mesh generator to handle these cases will be needed anyway.
• The support for setting the length of a capsule is a horrible kludge which is hard on the mesh cache — each length:radius ratio is cached separately. The idea I have for fixing this is adding “bone” support — the ability for selected vertices to be displaced by an externally controlled offset. This allows taking a single capsule mesh and stretching it without distorting the endcaps. (Tapered capsules will require multiple meshes to account for the angle at which the cone meets the endcap, but those can be fairly well approximated by a finite set of meshes.)

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