Fix decoration rendering with scale factor < 1

QPainter won't let paint with a device pixel ratio less than 1. There
are used to be workarounds to force a device pixel ratio of 1, but they
were removed with fractional scaling corner fix.

This change makes sure that the decoration renderer forces a device
pixel ratio of 1 if the output's scale factor is less than 1 so
calculated texture coordinates match where window borders are rendered
in the texture atlas.

BUG: 449681