layout: Rewrite text and inline fragment handling during flow
construction to avoid cloning and moving flows so much.

Besides amounting to a 5%-10% win on a page with a lot of text, this
simplifies and refactors the text layout code.
layout: Don't destroy the flow tree when resizing the window
gfx: Make fonts that fail to match hit the cache too.

50%-100% layout perf improvement on CNN.
gfx: Don't needlessly push and pop clip rects all the time.

15% painting improvement on CNN.
auto merge of #3722 : pcwalton/servo/flow-construction-overhaul, r=glennw

This is a grab bag of performance improvements that significantly improve style recalculation, layout, and painting on a few static pages.

Let me know if you'd like me to split this PR up.

r? @glennw