Add glTF-style animation support to WebGL displays

[dmurdoch]

glTF files contain support for animations in two ways: matrices in nodes (i.e. subscenes) can be animated, and individual vertices can have their positions depend on more than one node. This should all be supported in the WebGL display, allowing controls like playwidget to control items more flexibly.

[dmurdoch]

This should be done with something like par3dInterpControl, but that function isn't well suited to do it. It needs:

• Use separate rotation, translation, scale parameters
• Don't bother going through the par3dinterp function
• Use the times supplied by the user