OpenXR: Add support for XR_FB_hand_tracking_mesh;
OpenXR: Add support for XR_FB_hand_tracking_mesh;

This adds the ability to load and animate a mesh for hand tracking on
the Oculus Quest.  It is more or less identical to the current
functionality on the vrapi driver.

One key part of this change is that getPose in OpenXR will see if action
spaces are active before locating their spaces.  This is due to some
behavior observed on the Oculus Quest with hand tracking  where pose
actions for controllers would return invalid data with all of the
location flags erroneously set.  The only way to detect and work around
this is to check the pose action state.  When this happens, we fall back
to returning the pose of the wrist joint, which is where the Oculus hand
mesh wants to be drawn.  In the event that both controllers and hand
tracking are active, the controller pose will be returned by getPose but
the wrist joint can still be accessed using getSkeleton.

Note that this does not yet include support for properly scaling the
hand mesh.

There are numerous opportunities for optimization here that may be
investigated in the future, though performance is well within an
acceptable range right now.

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