layout: Make the `LocalImageCache` `on_image_available` callback not use
`@`.

This is one small step toward parallel layout.
net: De-GC the image cache stuff.

This is a step on the way toward parallel layout.
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
auto merge of #1315 : pcwalton/servo/disable-parallel, r=metajack

To verify, turn ON optimizations and go to:

http://en.wikipedia.org/wiki/Yellow_River

r? @metajack