This is based on the border-radius.html demo page with text and most
assets removed. This has to be a screenshot based test as there's not
really something else that can be used for comparison.
This also makes the test a little incomplete as things like text
overflow clipping are not tested, but I'd like to avoid this test being
too brittle.
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.
