[css-backgrounds] Interpolate between layers of backgrounds as one cross-fade

[noamr]

This relates to #578.
Currently in css-images-4 we allow smoothly interpolating CSS images. However, in the case of backgrounds, often images are layered on top of each other, together with colors and background-blend-mode. In some cases we might want to cross-fade the whole background stacks, rather than animate each layer individually, which might not always be compatible (e.g. image-to-color).