background-clip has not kept up with element clip styles. Needs to be able to accept polygon, rect etc

[ExtAnimal]

It should be possible to use

background-clip : polygon(0 0, 100% 0, 100% calc(100% - var(--footer-height)), 0 calc(100% - var(--footer-height)));

Ans all other clip-path shapes.

[JoshuaLindquist]

This was previously proposed (#3140), but the conversation never started.

It's a niche design need, but I would have assumed I could already do this if I hadn't been reading otherwise for the last few minutes. It fits nicely with how clip-path and offset-path - though the naming of both of those makes me wonder if this should be background-path instead.

The naming of these isn't totally consistent. Both background-clip and mask-clip lack shape features, but work with the content/margin/padding box (etc.) instead. But clip-path and offset-path can do both at the same time.