VR Scene Inspection: Better way to calculate landmark angle from viewer

When using the "Add VR Landmark from Session" operator, we want to
calculate an angle around the Z-axis for the landmark to use as
ground-aligned base rotation. The former way to calculate this was
rather primitive and broke easily (angle would point in the opposite
direction).
Now we project the camera direction onto the ground plane to get the
angle. This should be a very reliable way to do it.

viewport_vr_preview.py

@@ -407,20 +407,40 @@ class VIEW3D_OT_vr_landmark_from_session(Operator):
     def poll(cls, context):
         return bpy.types.XrSessionState.is_running(context)
 
+    @staticmethod
+    def _calc_landmark_angle_from_viewer_rotation(rot):
+        from mathutils import Vector
+
+        # We want an angle around Z based on the current viewer rotation. Idea
+        # is to create a vector from the viewer rotation, project that onto a
+        # Z-Up plane and use the resulting vector to get an angle around Z.
+
+        view_rot_vec = Vector((0, 0, 1))
+        view_rot_vec.rotate(rot)
+        angle_vec = view_rot_vec - view_rot_vec.project(Vector((0, 0, 1)))
+
+        # We could probably use a 3D version of Vector.angle_signed() here, but
+        # that's not available. So manually calculate it via a quaternion delta.
+        forward_vec = Vector((0, -1, 0))
+        diff = angle_vec.rotation_difference(forward_vec)
+
+        return diff.angle * -diff.axis[2]
+
     def execute(self, context):
         scene = context.scene
         landmarks = scene.vr_landmarks
         wm = context.window_manager
 
         lm = landmarks.add()
-        lm.type = "CUSTOM"
+        lm.type = 'CUSTOM'
         scene.vr_landmarks_selected = len(landmarks) - 1
 
         loc = wm.xr_session_state.viewer_pose_location
-        rot = wm.xr_session_state.viewer_pose_rotation.to_euler()
+        rot = wm.xr_session_state.viewer_pose_rotation
+        angle = self._calc_landmark_angle_from_viewer_rotation(rot)
 
         lm.base_pose_location = loc
-        lm.base_pose_angle = rot[2]
+        lm.base_pose_angle = angle
 
         return {'FINISHED'}