Qt draws lines on half-pixel boundaries. Because of
rounding, this is not visible with non-scaled aliased
painting. However, on a high-DPI screen with scaling
enabled, the line under the menu bar is drawn at a one
pixel offset, which causes it to overlap the menubar items.

This change draws the line at the mathematically correct position.

Task-number: QTBUG-38858
Change-Id: I0a2260507d346b52a36258d479cde87064d991ef
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>