vtkSynchronizedTemplates2D relies only on InputArrayToProcess

* the `Scalars` array was still used to get Increments
(to navigate in the data array raw pointer)
leading to some inconsitencies when Scalars has a different
memory layout than the effective InputArrayToProcess

see issue #19386

Test was disabled due to ImageComparison (see #19221)
Reimplement it in C++ to use `CompareDataObjects` instead of rendering.