scene: Fix DecorationItem reacting to new geometry

Window::layoutDecorationRects() uses KDecoration2::Decoration::rect() to
get the bounding decoration rect.

While Decoration::rect() should normally match Window::rect(), they can
diverge for a brief moment during async geometry updates. The worst
possible case is that the cached item quads may not be invalidated when
the geometry updates saddle.

To fix that, make DecorationItem monitor decorated client size changes
instead of window frame geometry changes. The reason for that is that
Decoration::size() is effectively decorated client size with added border
margins.


(cherry picked from commit 857766eb)