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