Fixed a validation problem in QSvgIOHandler::canRead().
Fixed a validation problem in QSvgIOHandler::canRead().

QSvgIOHandler::canRead() only looked at the 80 first bytes in a SVG
file when looking for the <svg> tag. That tag can obviously be found
at an arbitrary offset from the start of the file, depending on
e.g. comments tags.

Task-number: 255419
Reviewed-by: Kim

igor-sfdc/qt-wk,igor-sfdc/qt-wk,KDE/qt,KDE/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,radekp/qt,RLovelett/qt,KDE/qt,RLovelett/qt,RLovelett/qt,RLovelett/qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,KDE/qt,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,RLovelett/qt,pruiz/wkhtmltopdf-qt,radekp/qt,KDE/qt,RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,KDE/qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,KDE/qt