decorations: Avoid opaque areas of decorations being larger than the rendered contents

Opaque is a QRegion in logical pixels, using .toRect will round to the
nearest integer in either direction. This can mean an area is considered
opaque outside the rendered area, leading to glitchy contents on
shadows.

This is most noticable on on X11 windows when fractional scaling is
used.

Long term I hope to move Item::opaque to QList<QRectF> and
WindowPrePaintData::opaque to device pixels.

(cherry picked from commit c6a3d76b)