Fix Aurorae decorations with non integer DPI

Summary:
As noted by Thomas L we're casting the scaleFactor to an int far too
early, which in the worst case means we end up with Aurorare themes not
rendering.

This moves the rounding to where it's used per border.

BUG: 380524

Test Plan:
Forced Xft.DPI to 95 with xrdb. Confirmed that it was broken
Applied this patch. Got decoration again

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: mart, rikmills, plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6099