Reimplement Animation using splines

Reimplement Animation using splines

Transform interpolation is now performed on individual components rather
than entire matrices.  This produces more accurate rotations and later
will allow animations to only affect some components of the transform.

Currently linear splines are used and the slope parameters are ignored.
Pose matrices are also disabled for now because I don't have any suitable
data for testing them.  The old implementation was broken in various ways
anyway.