Returns a value that transitions between start and end or %NULL if the
values cannot be transitioned.
So far, all implementations but numbers and rgba return NULL.
For now, we return FALSE for all default css values, so this is not very
useful.
I also think of this as an optimization equal, not a guaranteed equal,
because we don't even have a notion of what "equal" means.
For example, for background-repeat, "repeat, repeat" and "repeat"
are functionally equivalent. But the cssvalue has no idea that it's used
for background-repeat.
As a more complicated example, "repeat, no-repeat" and "repeat" are
equal to what one sees as long as there's only one image listed
background-image-source. But once you start transition'ing to an image
with 2 sources, it's different...
