auto merge of #1313 : pcwalton/servo/parallel-layout, r=larsbergstrom

This is one small step toward parallel layout.

r? @larsbergstrom
main: Disable parallel selector matching due to races.

To verify, turn ON optimizations and go to:

http://en.wikipedia.org/wiki/Yellow_River
gfx: Make the font context thread-safe.

This is one small step toward parallel layout.
main: Make the layout context sendable.