Prevent vtkAbstractTransform ModifiedEvent deadlock

The Update() method of vtkAbstractTransform uses a mutex lock, and calls virtual methods during the lock which are implemented by subclasses. Those methods can trigger callbacks that cause Update() to be called again, resulting in a deadlock.

By ensuring that calls to Modified() during Update() do not invoke ModifiedEvent, we can avoid at least some of these deadlocks.

See discourse.vtk.org/t/bug-in-vtkcamera-setuserviewtransform-with-an-inverse-vtktransform.