Fix text decoration width rounding

Fix text decoration width rounding

width passed in qreal should be set with setWidthF instead of
setWidth, otherwise the fractional part is lost.

Also remove the raster paint engine hack that has been removed in
Qt 4.8, it's no longer needed.

This fixed the drawUnderline and drawRightToLeft auto tests in
tst_qglyphrun.

Change-Id: Iace46fc27ff156c07fb4d08a145269fad36e9fdb
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>