Add new GL2PS testing system using ghostscript.
This commit replaces the md5sum based GL2PS tests, which were unreliable across GL implementations and difficult to maintain, with a ghost script based approach. Testing is performed as follows: 1) The unit tests generate a post script file 2) A CMake script, CMake/RasterizePostScript.cmake is called, which runs ghostscript to rasterize the postscript file into a png image. 3) A PNGCompare test is called, which uses new API in vtkTesting to read the png file back in and perform a fuzzy image diff. PNGCompare lives in the Rendering/GL2PS module. Change-Id: I0e99b903512166741fe572ce417c1d0d480c34cd
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