This commit fixes #16842 for vtkImageReslice.  If GenerateStencilOutput
is On, then stencil->InsertNextExtent() is called when vtkImageReslice's
execute method completes a row, but if a row has been split among more
than one piece, the stencil will be corrupted.  It's only when SplitMode
is "Block" that this is likely to occur.  The fix used here is to detect
when rows might be split, and to adjust the split mode so that it won't
occur if GenerateStencilOutput is On.