Add vogl_analysis.py script and enhance FS PP to enable NULL FS substitution

Vogl_analysis.py provided to do frame analysis.  See details in comment at top of vogl_analysi.py file.

Added FS PP Null capability to selectively modify FS to output constant color.
Added "--fs_preprocessor_prefix <path>/prefix" option to dump all in/out shaders to/from FS PP to specified location w/ given prefix.
Merge pull request #155 from tobin-lunarg/null_shader

Null shader and Frame Analysis capability