QQuickShapeGenericRenderer misuses qtVectorPathForPath()

qtVectorPathForPath() does not respect the const-ness of its argument as it replaces an internal pointer to the pathConverter. Since c7167509acdba006fe6da4ec1866acb214c18197 in qtbase this is a unique_ptr. Therefore, replacing it deletes the old one. Calling qtVectorPathForPath() concurrently from different threads without locking therefore leads to race conditions and crashes.

(before c7167509acdba006fe6da4ec1866acb214c18197 in qtbase it would still create a race condition, but then leak the pathConverter instead of crashing)