2x3 Matrices for affine 2d transforms

Hi,
I have added a new matrix, mat2d, which allows affine 2d transforms, like this: http://en.wikipedia.org/wiki/Transformation_matrix#Affine_transformations

So you get this form:
[a, b,
 c, d,
 tx,ty]
This is a short form for the 3x3 matrix:
[a, b, 0
 c, d, 0
 tx,ty,1]
The last column never changes and so operations can be shortened.

In fact pre v2.0 there was a mat3.multiplyVec2 which did something similar. Using a 2x3 matrix makes things shorter, particularly multiplies, inverts, rotations.

The downside of this approach some operations are not possible since it is not square (e.g. adjoint), and some other operations don't really make sense (e.g. transpose)


Anyway I have found it useful and just now updated it to work with v2. Have a look if you are interested :)