[effects] Repaint expandedGeometry in HighlightWindowsEffect

Summary:
The highlight windows effect tried to trigger repaints with the shadow
included by having an hardcoded pixel offset. This of course breaks if
the shadow is larger than the hardcoded value.

The reason presented on why it was done like that is no longer true and
in the effects the actual visible area including decorations and shadows
is available through the expandedGeometry.

BUG: 368495

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D2748