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.
Fixes bug when "clip" property does not affect abspos children.
This change makes "clip" property to be applied together with
"overflow: hidden" in `apply_clip_overflow_rect()` that already
handles abspos children correctly.