Expose user agent option to DOM navigator interface.
layout: Make content of `display: inline-block; overflow: hidden` visible.
auto merge of #3608 : glennw/servo/user-agent-navigator, r=Ms2ger

This also makes command line options available as a global. If we're happy with that change I will go through the rest of the code and update it to avoid passing and cloning the Opts structure.
auto merge of #3604 : pcwalton/servo/inline-block-overflow-hidden, r=glennw

Makes lots of GitHub appear.

r? @glennw
layout: Rewrite clipping to be per-display-item instead of having
a separate `ClipDisplayItem`.

We push down clipping areas during absolute position calculation. This
makes display items into a flat list, improving cache locality. It
dramatically simplifies the code all around.

Because we need to push down clip rects even for absolutely-positioned
children of non-absolutely-positioned flows, this patch alters the
parallel traversal to compute absolute positions for
absolutely-positioned children at the same time it computes absolute
positions for other children. This doesn't seem to break anything either
in theory (since the overall order remains correct) or in practice. It
simplifies the parallel traversal code quite a bit.

See the relevant Gecko bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=615734