Add an optimization for drawing simple borders.
Add an optimization for drawing simple borders.

This is primarily to speed up the time it takes to draw
large, simple borders on OSMesa. This makes some tests
run a lot faster when using software rendering, which
reduces the chances of timeouts occurring due to the
large number of tests that the Servo CI machines run
in parallel.

It's also a reasonable optimization for simple borders
when running a hardware renderer.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender,servo/webrender