cssimagescaled: Just return the computed image

cssimagescaled: Just return the computed image

... instead of returning either itself with uncomputed images or a
copy of itself with only one computed image and lots of other
uncomputed images that we're never gonna look at again.

This fixes expressions like -gtk-scaled(-gtk-recolor(...),-gtk-recolor(...))
which Adwaita uses for checkmarks and bullets.