Capsule shape support

[w0rm]

https://en.wikipedia.org/wiki/Capsule_(geometry)

Capsule shape is nice for modelling many interesting things.

Unlike the cylinder shape, it doesn't have to be approximated internally as a convex polyhedron with many vertices.

This makes it efficient for collisions.

TDB: api for Shape.capsule (because elm-geometry currently doesn't support Capsule3d)