Make the API expect a tranform of the proper category instead of
doing the check ourselves and returning TRUE/FALSE.
The benefit is that the mai use case is switch (transform->category)
statements and in those we know the category and don't need to check
TRUE/FALSE.
Using the wrong matrix will now cause a g_warning().
In particular, add a per-category querying API for the matrix:
- gsk_transform_to_translate()
- gsk_transform_to_affine()
- gsk_transform_to_2d()
- gsk_transform_to_matrix()
This way, code can use the relevant one for the given category.
