Today I spent six hours wrongly convincing myself that it's a driver bug
when vkGetPhysicalDeviceProperties2() is null on a 1.1 instance for a
1.0 physical device. It's not a bug, it's me not reading specs
carefully.
This commit thus basically moves all Instance-level extension-dependent
state to DeviceProperties, because it's actually device-dependent. Which
makes the DeviceProperties class quite heavy and thus it's good it was
readied to be transferred all the way to a Device instance a few commits
back -- I don't really want to do all the dispatch, string processing,
sorting and other mess more times than strictly necessary.
In addition, DeviceProperties::apiVersion() got renamed to version() and
a new isVersionSupported() API got added, mirroring what's on Device
itself; plus thanks to the chicken-and-egg problem of having to call
vkGetPhysicalDeviceProperties() twice, the device version and other
things can now be retrieved in a slightly more efficient way.
I'm a bit unsure why there's a device extension that actually gets used
on an instance and doesn't need any enabling (and thus there's
currently no way to disable it to test all code paths, hmm).