vulkan: add initial video common code.

Two sets of common code, one for parsing session parameters for h264/265 decode.

The second parses h264 slice headers, which might be needed by multiple drivers.

Turns out we need some more common code. there was a lot of pNext chasing in working out reference pictures, so just avoid that and pre parse them in common code.

anv needs to sort slice reference lists according to spec for P and B lists, so add support for that.