FrameSvg: Do not wreck shared mask frames

Summary:
When trying to update the maskFrame, there is a chance that it is already
shared by several instances of FrameSvg. In that case, do not wreck maskFrame
and instead act as follows:

• deref it
• try to lookup a matching frame in the shared frames
• if there is the matching frame, use it
• otherwise create a new one

Reviewers: Plasma, Frameworks, mart

Reviewed By: Plasma, mart

Subscribers: mart, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13384