QPalette::cacheKey() is different for same object iff own style sheet is used

Quoting QTBUG-112200, which has a reproducer for this issue, but was fixed in a different way:

I debugged the problem and found that QStyleHelper::uniqueName in QFusionStyle::qt_fusion_draw_arrow always gives different values on same objects. This is caused by palette.cacheKey() (only if our own stylesheet is used).

uniqueName() returning a different value every time should be fixed, too, because it makes the pixmap caching basically non-existent.

I've micro-optimized the painting, but it's of course still orders of magnitudes slower than blitting a cached pixmap.