A number of fixes to the matplotlib module.

* Remove PARAVIEW_USE_MATPLOTLIB (mpl is built when PARAVIEW_ENABLE_PYTHON
  is defined now)
* Add vtkRenderingMatplotlib as a conditional dependency to
  vtkExtensionsRendering
* Changes in VTK to allow building against older versions of python.

Change-Id: I2a2fcc59fcbbfb67645255773be6739f98a021a6
Merge topic 'MPLFix' into MPL-RemoveCMakeFlag

Conflicts:
	VTK

Change-Id: Ic8e1d67747b2654bd6107d924168ed47083f0422
Merge topic 'MPL-RemoveCMakeFlag' into next

947299c Merge topic 'MPLFix' into MPL-RemoveCMakeFlag
d955365 A number of fixes to the matplotlib module.