Make the Transform class a struct

Transform is a fairly simple type and I don't want to write
setters/getters for the underlying matrices.
Promote utility functions
Add matrix inversion code

Currently the code is taken from the blender source code and uses
Gaussian elimination with partial pivoting.
Add inversion code for transformations

We store the inverse transformation along in the Transform structure
(like pbrt) in order to have a trade-off between CPU cycles and memory
consumption.
Remove superflous empty line