gfx: Refactor the border drawing code and split out fragment display
list building into multiple functions.

This should have no functional changes; it's just code cleanup.
gfx: Fix a nasty bug whereby the transient clip was not always removed,
causing elements to be randomly clipped out.

Improves Wikipedia, Reddit, etc.
gfx: Clip the background properly when `border-radius` is used.

Improves Reddit, GitHub, etc.
auto merge of #4459 : pcwalton/servo/border-radius-clipping, r=glennw

Together these improve a large number of sites: GitHub, Reddit, Wikipedia, etc.

r? @glennw