This change adds support for `background-blend-mode` to Servo by
creating one stacking context per blend and a single stacking context to
serve as the blend container. This matches the approach that Gecko
takes.
Stylo PR: servo/stylo#344
Testing: This change causes some WPT tests to start passing. In general
this feature
is not well exercised by tests. I suspect that blending is hard to test
properly.
Fixes: #43621
Signed-off-by: Martin Robinson <mrobinson@fastmail.fm>
Co-authored-by: Martin Robinson <mrobinson@fastmail.fm>