[poppler] Fix for bug 30107

[Albert Astals Cid]

Hi, so bug 30107 describes a PDF in which PSOutputDev thinks it has to raster 
instead of converting to PS.

This happens because PreScanOutputDev says "transparency" even if all the PDF 
has is a transparency group with alpha value set to 1 (which is basically 
opaque).

I've created the following patch that is a bit less agressive with the 
transparency flag.

I've passed it over my regression test suite and seems to give improvements 
and no other problem.

Anyone against me commiting it next thursday?

Thanks,
  Albert
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