Both Chromium and Gecko delay the document's load event for CSS image
resource requests (background-image, mask-image, etc). We now start
fetching CSS image resources as soon as their stylesheet is associated
with a document, rather than deferring until layout. This is done by
collecting ImageStyleValues during stylesheet parsing and initiating
their fetches when the stylesheet is added to the document.
Fixes#3448
Instead of rendering a reference HTML page that wraps an <img> tag
pointing to a PNG, Screenshot tests now load the expected PNG directly
from disk and compare it against the rendered screenshot. This
eliminates the indirection of loading and rendering a second page just
to display a static image.
This also means --rebaseline now works for Screenshot tests, generating
the expected PNG automatically instead of requiring manual screenshot
capture and placement.
Changes:
- Add TestMode::Screenshot with its own collector and runner
- Move PNGs from Screenshot/images/ to Screenshot/expected/ with
normalized names matching input filenames
- Remove all 92 reference HTML wrapper files and the images/
directory
- Remove <link rel="match"> from all 94 Screenshot input HTML
files
- Update add_libweb_test.py Screenshot boilerplate accordingly
- Add Screenshot mode to results viewer image comparison tabs
