[Gui] fix HiDPI rounding bug
[Gui] fix HiDPI rounding bug

As reported in https://forum.freecadweb.org/viewtopic.php?f=10&t=56425&start=20#p485809
this PR fixes the problem reported here: https://forum.freecadweb.org/viewtopic.php?f=10&t=56425&start=20#p485759

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master