Ignore CSS zoom and text zoom for canvas.
The spec doesn't mention anything about applying them, and other
browsers don't, so let's just be consistent...
Differential Revision: https://phabricator.services.mozilla.com/D221709
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1909625
gecko-commit: 2de2533b5e622f7f6f75e6ad9ec773b1cf997cc8
gecko-reviewers: gfx-reviewers, lsalzman