Don't paint line-clamped lines.
This matches the proposal in
w3c/csswg-drafts#10816, and creates much better
behavior.
Differential Revision: https://phabricator.services.mozilla.com/D157578
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1791226
gecko-commit: cf9f54eef33465d089f850a55da5734a911fdac4
gecko-reviewers: layout-reviewers, dshin