Amends f9940b15f7, which
amended 6e1d70ae12.
The bevel of the button is not defined by the contentsRect, but by the
borderRect that the stylesheet style calculates.
Change-Id: I9a0d5bf29a06ce2270014f0d144e33cc3a1a7473
Pick-to: 5.15
Fixes: QTBUG-86587
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>