Don't tear down renderer and other busy work when Svg::setImagePath is invoked with the same arg

Summary:
During my plasmashell startup this saves time for 17 calls. Opening
something like Simple Menu saves another ~dozen.

It's worth investigating why Svg::setImagePath gets called repeatedly
with the same args to begin with, but this is worth doing anyway.

Reviewers: Plasma, davidedmundson, mart

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D9115