FrameSvg: also clear mask cache on clearCache()
Summary:
It can be unexpected that those are not reset when clearCache() was called.
With the current logic when a theme changes the mask cache not reset also
results in wrong lookups, as the cacheid used as key does not include the
theme name.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D21172