We should create a stacking context for any transform or opacity animations
that are either "in effect" (what we currently do) OR "current", i.e.
scheduled to run or running. *BUT* for now, we don't create any stacking
context in before phase without fill:backwards or fill:both because the
property never wins in cascade until the animation gets "in effect". This
restriction will be removed in a subsequent patch in this bug after landing
bug 1279403.
MozReview-Commit-ID: 8RyLJNPtoKI
