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.
cc @juliuszint