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
1 job for css-image-surface-draw-with-provider-scale in 10 minutes and 39 seconds (queued for 22 seconds)
Status Job ID Name Coverage
passed build #20131