Use that instead of hacks to guess when an image is considered invalid
according to https://drafts.csswg.org/css-images-4/#invalid-image
Also add a GtkCssImageInvalid that implements the behavior of invalid
images according to the CSS spec so thjat image implementations can
refer to that image.
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.
This lets us do fallback in case an image format is not
supported, and also lets us provide solid-color images.
We don't support image fragment notations.
See ttps://www.w3.org/TR/css3-images/#image-notation
https://bugzilla.gnome.org/show_bug.cgi?id=761318