Crash was caused since the function pointers
`s_xrGetOpenGLGraphicsRequirementsKHR_fn`/
`s_xrGetD3D11GraphicsRequirementsKHR_fn` were static and were not
updated with the correct proc address after being set the first time.
As stated in the OpenXR spec: "function pointers returned by
xrGetInstanceProcAddr using one XrInstance may not be valid when used
with objects related to a different XrInstance".
Although it would seem reasonable that the proc address would not
change if the instance was the same (hence the `static XrInstance s_instance;`),
in testing, repeated calls to `xrGetInstanceProcAddress()`
with the same instance still can result in changes (at least for the
SteamVR runtime) so the workaround is to simply set the function pointers
every time, essentially trivializing their `static` designations.
Reviewed By: Severin
Maniphest Tasks: T94268
Differential Revision: https://developer.blender.org/D14023
4fbd00e04c