We slightly expand the syntax of -gtk-scaled to allow
specifying an explicit scale after each image, and then
we create a single-image GtkCssImageScaled with the
preferred scale in compute().
This is just lots of renaming.
The interface remains private, so the public API does not change, apart
from removing the definition of the Interface object to avoid
subclassing.
... 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.
