[x3d-public] HAnim and glTF skins

[Andreas Plesch]

I figured out by now that these fields were added for HAnim 2.0
whereas X3D V3.3 uses HAnim 1.0 .

I think the idea is that you have a skeleton of joints with animation
data designed for it but the actual initial geometry is designed for
another initial state (pose). Then you can either redo the skeleton
and animation, redo the geometry, or apply a first ('binding')
transform to get the geometry into the state expected by the animated
joint transforms. This first transform would be defined by the
jointBinding fields, or as an inner joint in the skeleton's joint
hierarchy.

I think these fields are similar to glTF's inverseBindingMatrix which
is also defined for each joint, and is applied to initial vertex
positions. So in order to play glTF skinned animations, it would make
more sense to implement these fields rather than finding a singular
solution.

It is unclear to me how skinCoord and skinBindingCoord interrelate,
and why non-humanoids are supposed to use those. Probably because it
is common to have this initial binding step rather than compatible
skeletons and geometry in the first place.

Hopefully somebody can correct all the mistakes and misunderstandings
just offered above :)

-Andreas