Slider: Rework sizing and positioning to draw them centered

If you stretch in its orthogonal direction (vertically for horizontal
and vice-versa), a Slider with Breeze style will now act similar to
Fusion style: it will center itself within given rectangle. But unlike
Fusion, it would also account for Breeze specific tick marks which are
so "small" from QStyle point of view that we have to subtract 2px back.

BUG: 474199