Support OpenVG graphics systems that inherit from QVGPixmapData
Support OpenVG graphics systems that inherit from QVGPixmapData

Custom graphics systems, may want to inherit from QVGPixmapData to
implement alternative pixmap management strategies.  We make toVGImage()
overridable and the data members protected to support this.

Reviewed-by: Sarah Smith
(cherry picked from commit 71d05ae1f212793b59c2f0fcb68ec5f21732073e)

pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk