Each ref test now links to its reference page with a link tag, in the
same format as WPT:
`<link rel="match" href="reference-page.html" />`
The reference pages have all been moved into a separate `reference/` dir
so that we can just treat every file in `ref/` as a test. There's no
filter to only look at .html files, because we also have a .svg file in
there, and there may be other formats we want to use too. But it's not
too hard to add one if we need it.
We still don't know how to resolve font-relative lengths in <img sizes>
since we don't always have font size information available at this stage
in the pipeline, but we can at least handle viewport-relative lengths.
This fixes an issue on many websites where low-resolution images were
loaded (appropriate for a small viewport) even when the viewport is big.
