Check if transforms are affine

[javagl]

Brought up via CesiumGS/3d-tiles#711:

The specification says that each transform must be an affine matrix.

The validator currently only checks whether a matrix is the zero-matrix. But it should check whether it is affine (i.e. whether the last row is (0,0,0,1), up to an epsilon).

(Whether or not non-invertible matrices should be allowed is open for debate, but checking them for being affine at least matches the current specification)