kstyle: fix qqc2 desktop style sliders in RtL

We don't do any mirroring of the slider groove rects or tickmarks based on layout direction,
and this causes qqc2-desktop-style sliders to render in the wrong direction.
Swapping the leftRect and the rightRect when option->direction == Qt::RightToLeft
fixes this.

This wasn't broken with QWidgets because QSlider relies on (ab)using the upsideDown option
in order to achieve mirroring of the groove instead of option->direction, and consequently,
this change doesn't affect Qt widgets apps at all.

BUG: 430101