Fix pixelgrid snapping of native text on retina displays.

Change 63e6c9ad

 introduced snapping to
the pixel grid in the vertex shader for native text, but this code was
broken on retina displays because it assumed integer only positions.
Fix it by including the retina scale factor into the rounding.

Task-number: QTBUG-38702
Change-Id: I84492b02d64f263c9fe030790e04cf79b0dc4e2f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>