Allow an image reader to read data from buffer instead of file.

This feature may be very useful to avoid the serialization of a temporary
image before reading it with a VTK reader - for instance an image fetched
from an internet socket or any procedural fashion...

The 2 buffer related variables (MemoryBuffer and MemoryBufferLength) are
implemented in vtkImageReader2.
For now, only the JPEG reader supports this new feature.

Change-Id: I9d00e19c92143cd4190353ccabf574012696d6db
Fix segfault in vtkMatplotlibMathTextUtilities destructor

Several of the ParaView Python tests fail with a segfault in the
destructor of the vtkMatplotlibMathTextUtilities when it calls
Py_Finalize(). This changes the destructor to only call Py_Finalize()
if vtkMatplotlibMathTextUtilities itself called Py_Initialize().

Change-Id: I7fab0273a65c6e6784f1acf4266dfc53594d5fca
Merge topic 'AddSupportForBufferImageReaders' into master

579e7a7b Allow an image reader to read data from buffer instead of file.
Merge topic 'fix-matplotlib-math-text-segfault' into master

169c9bf5 Fix segfault in vtkMatplotlibMathTextUtilities destructor