Expose the display list to consumers of embedded servo

[philocalyst]

Exposes the list so that consumers can handle rendering themselves, like for a... TUI webbrowser -- mainly just plumbing. There's a teeny bit of logic for helpers in such that could likely be improved/have the greatest impact of this PR being effective, so I would specifically check out here and here. Also didn't know about the color type to expose, I went with ColorF.

Testing: Adapted for use in a private branch of carboxyl, all tests pass.
Fixes: General unvoiced desires within certain enclaves of embedder folk.

[mrobinson]

@philocalyst Did you mean to submit this upstream?

[philocalyst]

@mrobinson Now I did! Sorry for the confusion. I am NOW reading the servo contribution book so expect some changes from here.

[mrobinson]

A few high level issues that need to be worked out before I'm ready to accept an change like this:

1. This change does not properly handle reference frames and scroll nodes, so might work for simple pages, but likely not complex ones.
2. This change does not handle subframes. That might work for a simple usecase, but not for complex ones.
3. Only the root scroll frame offset is considered, but not other scrollers.

This is an interesting proof-of-concept, but a feature like this needs careful consideration. The problem with landing this as-is is that I (and others) will need to maintain it and also be responsible for adding missing features and filling in the blanks.

I think a good way to go about this would be to start with a high-level API proposal that considers all the issues listed above -- and probably some that I've neglected to spot on a first look. It probably makes sense to spend a bit of time getting familiar with the internals of Servo first. A good way to start is to try to fix a few bugs while working on your API proposal.