There are a few canvas2d-related dependencies that haven't updated, but they
only use euclid internally so that's not blocking landing the rest of the
changes.
Given the size of this patch, I think it's useful to get this landed as-is.
Have background, border and gradient modules
for calculation functions.
Use shorter names for functions that are qualified
by the module name like `border::radii`.
Use push_item and push_iter to add items to WebRender.
