FrameSvgItem: emit maskChanged also from geometryChanged()

Summary:
Needed e.g. to fix the missing update of the mask on changing the size of
the Plasma panel if only listening to mask changes.

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21201