gfx: Clip the background properly when `border-radius` is used, and clean up some painting stuff.

[pcwalton]

Together these improve a large number of sites: GitHub, Reddit, Wikipedia, etc.

r? @glennw

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3565

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[pcwalton]

r=glennw

[bors-servo]

saw approval from glennw
at pcwalton@cc7cacf

[bors-servo]

merging pcwalton/servo/border-radius-clipping = cc7cacf into auto

[bors-servo]

pcwalton/servo/border-radius-clipping = cc7cacf merged ok, testing candidate = 63a7742

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/390
success: http://build.servo.org/builders/linux2/builds/389
success: http://build.servo.org/builders/android/builds/425
success: http://build.servo.org/builders/mac1/builds/606
success: http://build.servo.org/builders/mac2/builds/607

[bors-servo]

fast-forwarding master to auto = 63a7742