[effects/snaphelper] Use windowFrameGeometryChanged

Summary:
Use windowFrameGeometryChanged rather than windowGeometryShapeChanged
because we are interested only in frame geometry changes.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26909