Before this patch, we would use fallback for all border images. Now for
all but vector images we will use the WebRender border images
primitives. Vector images are an exception because the fallback is
clever in that it upscales the vector image and clips to only draw the
region it requires. This avoids artifacting but to do something similar
for WebRender as it is currently defined, we would increase our CPU and
memory footprint as we would need to produce the entire vector image
upscaled, not just the parts we need. In the future we should change
WebRender to accept different image resources for each of the segments.
Differential Revision: https://phabricator.services.mozilla.com/D37093
