Qt 4.8 shows frequent crashes in runMultiple apparently caused
by the QMutex construction in the free functions by different
threads. Use a common QMutex class member instead.
Change-Id: I851d4e2d3637a7b4f404ed843f5360c10caa21f5
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>