cssimagesurface: scale cache using the css provider scaling
Currently the image surface device scale is set to the same value of the target device scaling, but that value could not be set or having different value when used outside gtk itself (like in mutter decorations).
We should instead use the same scaling of the css provider as it's the same that -gtk-scaled already uses.
So this needed the compute() method to be implemented inside GtkCssImageSurface and make sure that GtkCssImageUrl and GtkCssImageScaled were computing the image surface they initialize.
Fixes #148 (moved)