When turning on VR mode, if no controllers are available,
a fallback model (a sphere) is rendered in the middle of the VR area.
This change uses the OpenXR API to check if the controllers are
available, sets new properties in `GHOST_XrPose` and `wmXrController`
that store if each controller is active or not, and uses
those properties to render the controllers in the VR view only
when they are active.
NOTE: in `wmXrController`, two state properties have been included,
but only `grip_active` is currently used. `aim_active` has been
included for consistency, but it could be removed.
Pull Request: https://projects.blender.org/blender/blender/pulls/119989
821b773d95