cssvalue: Add _gtk_css_value_equal()
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...
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