This should make the optimization landed earlier in this bug apply for
some of the NotifyThemeChanged() calls in nsWindow.cpp which are causing
all the extra invalidations.
If we know that system colors/fonts didn't change, we can avoid doing a
bunch of reflow work and the patch from earlier in the bug can avoid
re-rasterizing images too.
Differential Revision: https://phabricator.services.mozilla.com/D94425
Emilio Cobos Álvarez