Since getFrames() must gather all properties set at a given keyframe
offset time for a given easing function, we need to provide a total
ordering for ComputedTimingFunction objects. Until the spec defines how
to do this, we sort first by NS_STYLE_TRANSITION_TIMING_FUNCTION_*
value, then second by the four values in a cubic-bezier() function (in
order) or the integer and optional keyword in a steps() function.
Because we don't support automatic spacing of keyframes yet,
ComputedKeyFrame.computedOffset is always the same as Keyframe.offset.
Another assumption made is that the value of easing for a Keyframe
object at 100% should be the same as the value from the previous
Keyframe for the same property. An alternative would be to leave off
easing from that Keyframe, which would need the default value for that
IDL dictionary member removed (otherwise it would always be set to
"linear").
