[vtkusers] Fast conversion from vtkImageData to QImage?

[John Drescher]

>  The only way faster would be to try and work with the raw pointers for both
> the vtkImageData and the QImage. You do lose lots of safety checks, but from
> my experience calls like scalars->GetTupleValue(tupleIndex++, tuple); take
> longer due to the bounds checking (which is good) but if you feel you can
> get along without the bounds checking (because you already have checked) you
> can do it that way.

On top of raw pointers I would have specialized a case for each type
in the switch statement instead of executing the switch for every
single pixel. However raw pointers probably require that anyways.

John