RFC: vulkan: Common GetPhysicalDevicesFeature2

Flatten out all the extension structs into a vk_features (since most of
them are just a few booleans, and all of them are appropriately
namespaced anyway) and then the switch statement + parsing can happen
in a common helper.

Potentially the core features structs should also be flattened out for
consistency. And then maybe we might want to alphabetize features? IDK
how much value the versioning history gives.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>