Fixed vtkImageGradient in parallel.

vtkImageGradient did not work in parallel because it assumed that
the input extent is the same as the update extent, which is not
the case when there are multiple pieces requested. This bug is
prevalent among the imaging filters but for now I am fixing them
one at a time. Fixing them all at once is non-trivial.

Change-Id: I28bdd56e46c97ef7a80e18a46b6d266fbcd79134
Merge topic 'image-gradient-bug-14982' into master

869c2d1a Fixed vtkImageGradient in parallel.