[poppler] Generic PNM support

[Pino Toscano]

Hi,

currently, there's only SplashBitmap able to export to PBM/PGM/PPM.
I just created a PNMWriter to provide generic export capabilities, even 
when not using Splash(Bitmap) directly.
References:
- PBM: http://netpbm.sourceforge.net/doc/pbm.html
- PGM: http://netpbm.sourceforge.net/doc/pgm.html
- PPM: http://netpbm.sourceforge.net/doc/ppm.html

Attached there's a small patch for pdftoppm to use the new export 
filter, instead of the own SplashBitmap PNM exporting method.

-- 
Pino Toscano
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