Add support for <super> and <sup> tags. Fixes equations on wikipedia.
auto merge of #3523 : glennw/servo/wiki-equations, r=pcwalton
Pass a viewport to the rendering code

Part of servo/rust-layers#114
auto merge of #3525 : mbrubeck/servo/viewport, r=zwarich

Updates to the latest rust-layers and rust-geom to pick up
servo/rust-layers#114.  r? @zwarich
Mask root layer rectangles to their boundaries

This prevents iframes contents from overflowing their root layers.
auto merge of #3526 : mrobinson/servo/masks-to-bounds, r=zwarich

This prevents iframes contents from overflowing their root layers.