Throw an error if two animations attempt to control the same node
Go through all assimp animations when splitting frames
Merge remote-tracking branch 'luomu/anim-fixes'
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