Clear generated content when frame is reset
Clear generated content when frame is reset

CSS-defined content is created as children within a generated node during the reflow process. When a node is paged it will go through the reflow process again. This change removes any children from a generated node to prevent content duplication.

addresses #1404
addresses #1434
addresses #1187

nathanntg/dompdf,dompdf/dompdf,BrunoDeBarros/dompdf,dompdf/dompdf,nathanntg/dompdf,BrunoDeBarros/dompdf