gfx: Add display list optimization to eliminate clipped items before
submitting them to Skia.

40x improvement in painting time on Wikipedia.
auto merge of #2588 : pcwalton/servo/display-list-optimization, r=glennw

submitting them to Skia.

40x improvement in painting time on Wikipedia.

r? @glennw
Fix the vertical alignment error for img tag.
auto merge of #2592 : june0cho/servo/fix_vertical_align, r=SimonSapin

It fixes the wrong vertical alignment for img element. (related to #2422, #2554)
Worked with @ryanc
Implement TElement for JSRef<Element>
Implement TNode for JS<Node>
Added ParentNode.querySelector skeleton
Implement querySelector for Document
Implement querySelector for DocumentFragment
Implement querySelector for Element
Added content tests for querySelector