cssimagefallback: Don't compute new image if only a color is set

cssimagefallback: Don't compute new image if only a color is set

Themes might use e.g. image(red), which is a constant value and will
never change. In that case, the fallback image has ->color set, but not
->images. If that's the case and the computed color is the same as
the one we already have, just return the already existing image.