Update `render' functionality:
    
        - Move post-rendition and last-rendition invocations from
          `render_doBaseActions.sh' to backend-specific scope. This way it
          is possible to reuse variable definitions inside backend's
          scope. Otherwise it wouldn't be possible doing so.
    
        - I needed to use the EXPORTID variable inside Svg post-rendition
          actions and that can happen, only if post-rendition actions are
          inside Svg backend's scope.
    
        - Docbook backend was updated for consistency with Svg backend.