Fixes for multithreaded bug.
SimdXContext::_fileName was a reference to a static std::string.
Two threads, which each have their own SimdXContext, could simultaneously
try to assign a new value to _fileName, meaning they would simultaneously
try to assign a new value to the same static string.  In this case the
assignment operation in each thread would attempt to free the storage for
the old value and you would get a "double free" error.