gfx: Implement `box-shadow` per CSS-BACKGROUNDS.

r? @SimonSapin 
Test failure: http://build.servo.org/builders/linux1/builds/301/steps/shell_1/logs/stdio

Indentation makes Markdown parse this as a code block, which rustdoc apparently defaults to parse as Rust code for a doc test. Try an explicit code block with a non-Rust type annotation like ```` ```text ````