Extended enums

[msiglreith]

Enums in Vulkan can be extended due to extensions and the Vulkan backend might return such extended values, resulting in panics on our side as we don't know of these (possible future) enum variants. KhronosGroup/Vulkan-Docs#656

A future driver update may result in crashes on our portability implementation under the assumption that we will have to distribute a vulkan backend portability implementation instead of the normal vulkan driver implementation.

The #[non_exhaustive] attribute might be useful here when it is implemented.