New3dapi: various changes, including animation and skinning

I'm still not ready cleaning up things etc. but I don't want to have too much changes at once, so here are the changes so far.

It also contains some other changes (I am rewriting DefaultShader to be more readable, include spherical harmonics, etc).

I added the knight g3dj and g3db model for testing.

I updated the fbx-conv PR accordingly https://github.com/libgdx/fbx-conv/pull/3