Fix#4319. PR #3495 allowed Mpdf and Tcpdf to handle hidden rows and columns. The handling is a little off when merged cells are part of the output. The fix for Mpdf is pretty straightforward. Tcpdf is a little more convoluted. Dompdf and Html do not require a change. Unit tests are added for the same scenario for all of Mpdf, Tcpdf, Dompdf, and Html.