Fix problem reading buffer from matplotlib

matplotlib.backends.backend_agg.FigureCanvasAgg.print_to_buffer()
prints an image to an old-style Python buffer object and ParaView was
using the API for the new-style Python buffer object. Serendipitously,
this works in Python 2.7.5 (on my development machine), but that is
not so on other Python versions. Switched to using the API for the
old-style buffer object and verified that it works with Python 2.7.2.

Change-Id: Idfa91c27109756e275d690229865e5f648249bda
Merge topic '14491-fix-matplotlib-buffer-object-access' into next

505bff5 Fix problem reading buffer from matplotlib