vulkan: allow directly wrapping VkDevice for interop

Rather than having libplacebo create it. Some API users really want this, in particular ffmpeg assumes the hwcontext_vulkan will be creating the vulkan device, so API users need to have libplacebo piggyback off of it.

There's some unfortunate amount of logic duplication between this function and pl_vulkan_create, but I'm not sure if trying to factor this out into a common function is worth the effort it saves. It'll probably end up just as ugly.