avoid truncation of zoom factor used for DPI scaling

This was originally reported and already fixed upstream:
https://bugs.webkit.org/show_bug.cgi?id=40662

However, when porting c1af97f5f73477f62427bdf0b6e1480b6f75b6f3 to
the latest Qt version this fix was lost (due to code moving around).

wkhtmltopdf/qt#12