Fix subgrid + virtual scrolling

Fixes #545

Clear demand for this-- see:
 * http://stackoverflow.com/questions/7814927/jqgrid-subgrid-stops-opening-after-scroll
 * http://stackoverflow.com/questions/22307727/jqgrid-subgrid-not-expanding-after-scroll
 * http://www.trirand.com/blog/?page_id=393/bugs/problem-with-subgrid-and-scroll/