This is a projecting version of the corresponding
graphene api. While we are at it, rewrite
gsk_matrix_transform_bounds() to use
gsk_matrix_transform_rect().
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.
