gtk: Add GtkTransform

gtk: Add GtkTransform

This is a new object (well, boxed type, but I'm calling it object) for
dealing with transform in a more constructive way than graphene_matrix_t
by keeping track of how the transform was created.

This way, reasoning about the transform becomes easier, and we can create
better ways to print it or transition from one transform to another one.

An example of this is that while a 0 degree and a 360degree rotation are
both the identity matrix, doing a transition between the two would cause
a rotation.