Avoid use of shared renderer across threads

David Edmundsonrequested to merge
work/d_ed/thread_svg into master

SharedSvgRenderer inherits QSvgRenderer. QSvgRendered is re-entrant, not thread safe. Therefore we should not share instances between threads.

After this the mutex is not needed.

Fixes: https://crash-reports.kde.org/organizations/kde/issues/205523

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