Implement CPU rendering. Replace texture sharing with native OS surface sharing.
auto merge of #553 : pcwalton/servo/cpu-rendering, r=pcwalton

r? @metajack
Move special cases of CSS 'display' the property to computed value, per spec.
Use Au::scale_by instead of duplicating it.
Fix selector parsing.
Move CSS21 pseudo-elements parsing out of parse_simple_pseudo_class