delete a load of unused animation time control code

Models and animations need some reworking anyway to separate per-model data
(meshes, materials, animation hierarchy, keyframes) from per-instance data
(pattern/decal/label/effect state, animation state, cached transforms).
Reworking stuff is usually easier when there's less stuff to rework, so
therefore deleting stuff is good (right?)
Remove looping capability from Animation::Interpolate()
Change scale & position animation too
Merge animations across meshes

Conflicts:
	src/scenegraph/Loader.cpp
convert keyframe times to seconds during load, and remove keyframe offset
allow individual keyframes (e.g., some models have just one scale key)
when merging animations, use the longest duration

In practice, you most likely want all merged animations to have the same
duration. But if they don't, at least this code means the result isn't
dependent on the order in which you specify your animated meshes.