Support different error metric shapes

[nfrechette]

Currently the compression error is evaluated at 3 points on the surface of a sphere centered on the animated joint.
See here.

While this works fine and it is simple to implement and fast to execute, it can often poorly approximate the visual mesh skinned on the joint. Other shapes are often used for this in physics simulations such as: capsules, boxes, cylinders, convexes, etc.

This could be supported per joint and would allow a game engine to use the physics asset associated with a skeleton.