Move the SharedResourceRequest, animation timer, and current frame
state out of ImageStyleValue and into a Document-owned table keyed
by resolved image URL. ImageStyleValue now keeps only URL metadata
and its client list, so image style values no longer need to trace
GC edges themselves.
Thread the Document through AbstractImageStyleValue APIs that need
decoded image data. CSS image fetches snapshot the stylesheet base URL,
referrer behavior, and origin-clean state instead of retaining the
stylesheet.
Remember each client's registered resolved URL when unregistering. This
keeps a later document base change from leaving an animated image
resource alive.
Add text coverage for inline relative image base URLs, stylesheet
referrers, imported stylesheet origin-clean behavior, inline @import
initiator type, and unregistering an animated background image after a
base element change.
url() has some limitations because of allowing unquoted URLs as its
contents. For example, it can't use `var()`. To get around this, there's
an alternative `src()` function which behaves the same as `url()` except
that it is parsed as a regular function, which makes `var()` and friends
work properly.
There's no WPT test for this as far as I can tell, so I added our own.
Our previous approach to `<url>` had a couple of issues:
- We'd complete the URL during parsing, when we should actually keep it
as the original string until it's used.
- There's nowhere for us to store `<url-modifier>`s on a `URL::URL`.
So, `CSS::URL` is a solution to this. It holds the original URL string,
and later will also hold any modifiers. This commit parses all `<url>`s
as `CSS::URL`, but then converts it into a `URL::URL`, so no user code
is changed. These will be modified in subsequent commits.
For `@namespace`, we were never supposed to complete the URL at all, so
this makes that more correct already. However, in practice all
`@namespace`s are absolute URLs already, so this should have no
observable effects.
