[gl2ps] bug for opacity=0.5 and TextAsPath=1

[Dan Lipsa]

Hi all,
We have a merge request in VTK which includes a fix to gl2ps.

https://gitlab.kitware.com/vtk/vtk/merge_requests/2277

The test we use is:
https://gitlab.kitware.com/vtk/vtk/blob/6d51efa1f268b7cec5dc0ce1e195db4f1c812a23/Rendering/GL2PS/Testing/Cxx/TestGL2PSTextOpacity.cxx

If we set for text the opacity to 0.5 and TextAsPath to 1 we get an invalid
PDF (text_bad.pdf). After our fix we get an valid PDF (text.pdf). See
attached files.

Would you please review our changes and once approved include them in
future versions of gl2ps.
Changes to a2ps are listed as the last two files in the list of changes:

https://gitlab.kitware.com/vtk/vtk/merge_requests/2277/diffs


Thank you,

Dan
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