snapshot: Store an actual GtkTransform

snapshot: Store an actual GtkTransform

Instead of just tracking 2 integer translate_x/y coordinates, tracka a
full GtkTransform.

When creating actual nodes, if the transform is simple enough, just
create the node in a way that makes use of the transform. If the
node, can't represent the transform, just push a transform node instead
and automatically pop that node with the next gtk_snapshot_pop() call.