This change introduces a new system for providing extra instance/device extensions.
This consolidates platform-specific and vr-related to its' own thing for potential future cross-platform/native Linux work.
Adding an extension will now require an additional step, but
this evolved version of the extension list code is more sane
and the structures can be more easily copied around.
Preserves the per-instance and per-adapter extension sets across
multiple Vulkan instances. Works around initialization order issues
with Unity-based VR apps.
