unrendered content uses doc's background color

  uses html's background color, or body's.
auto merge of #1210 : ibnc/servo/resizing_background_color, r=jdm

fixes #140

This shouldn't be merged until the appropriate change is made in rust layers (https://github.com/mozilla-servo/rust-layers/pull/37). I'm making a pull request anyway to start getting some feedback.