It's a GNU extension part of glibc since 2.17, and it was also added
recently to musl libc. It doesn't seem to be available on *BSD (but
also not used there by Vulkan-Loader).
Could be made more thorough by doing a test compilation of a file to
check for the existence of the function on the host system, but unless
we run into actual issues, that's likely overkill.
- Renamed option to `builtin_vulkan`, since that's the name of the
library and if we were to add new components, we'd likely use that
same option.
- Merge `vulkan_loader/SCsub` in `vulkan/SCsub`.
- Accordingly, don't use built-in Vulkan headers when not building
against the built-in loader library.
- Drop Vulkan registry which we don't appear to need currently.
- Style and permission fixes.
-Added VulkanContext
-Added an X11 implementation
-Added a rendering device abstraction
-added a Vulkan rendering device abstraction
-Engine does not work, only shows Godot logo (run it from bin/)
