Clip display list based on frame viewport

Instead of creating a display list for the entire page, only create one
for an area that expands around the viewport. On my machine this makes
incremental layout of http://timecube.com 50% faster.
Move code related to json packet reading to JsonPacketSender trait

fixup! Move code related to json packet reading to JsonPacketSender trait
Fix #3933
auto merge of #3955 : thiagopnts/servo/devtools-refactoring, r=jdm

I was messing around devtools code and saw some TODOs, is anyone working on it? I took one of them:

```// TODO: this really belongs in the protocol module.```

I would be glad to help with this if no one is on it already, just let me know.
auto merge of #3938 : guillaumebort/servo/fix/3933, r=jdm

Implement HTMLOptionElement.{label,value}
auto merge of #3809 : mrobinson/servo/display-list-optimization, r=pcwalton

Instead of creating a display list for the entire page, only create one
for an area that expands around the viewport. On my machine this makes
incremental layout of http://timecube.com 50% faster.
[gfx/style] Implement border-radius.

This patch is a first stab at implementing border-radius. It looks fine as long as
the border isn't an ellipse (that might not even parse yet), and the border-widths
around a border-radius are the same.

Here's a cool screenshot!

![](https://www.dropbox.com/s/gdtmgjrlnf82gzz/Screenshot%202014-11-12%2018.03.29.png?dl=0)