... and Make this new value be a real GValue, as we don't need to save
performance for these anymore (it's just used for custom properties).
And I'd rather have code work for all values then be optimized for no
reason.
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.
Note: custom CSS properties still use the default GtkCssValue and always
will.
So there is a difference in css values used between those, even though
they both carry a GdkRGBA payload.
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...
