Fix: Incorrect VR pose after changing landmark

Once the base pose was changed (e.g. by changing the active landmark), we'd always run the logic to reset to the base pose. That would mess up the final viewer pose.
Think this only got exposed through 607d745a79.

source/blender/windowmanager/xr/intern/wm_xr_session.c

@@ -278,6 +278,8 @@ void wm_xr_session_state_update(const XrSessionSettings *settings,
   state->prev_base_pose_type = settings->base_pose_type;
   state->prev_base_pose_object = settings->base_pose_object;
   state->is_view_data_set = true;
+  /* Assume this was already done through wm_xr_session_draw_data_update(). */
+  state->force_reset_to_base_pose = false;
 }
 
 wmXrSessionState *WM_xr_session_state_handle_get(const wmXrData *xr)