Add groove and ridge border support.
Add ref test for dashed, dotted, groove, and ridge borders.
Don't line up :'s in render_context.rs (style)
Refactor groove & ridge border support, add inset & outset border support
Significant cleanup of border support, adds inset & outset border code. Border rendering matches
Chrome's border rendering.
Cleanup whitespace
Cleanup whitespace + move functions for easier reading.
Double border support
Replace tuple with Rect<f32>
Also includes other minor code cleanup.
auto merge of #2465 : bjwbell/servo/groove-ridge-borders, r=pcwalton

Matches Chrome's rendering behavior for groove and ridge borders (I can attach comparison images, if wanted).