Remove a redundant full screen rectangle being drawn each frame.
Remove a redundant full screen rectangle being drawn each frame.

The root pipeline doesn't need to draw a rectangle for background
color, as this is handled by the framebuffer clear.

In some pages, this saves ~0.4ms GPU time per frame for the test
setup I am using (Intel HD530 @ 2048x2048). Other pages typically
see a gain of around 0.2ms / frame.

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