Makes gcc 12.1 happier, i think it was a gcc bug anyway, but the new
code is a bit "better" anyway, the warning i was getting is

In file included from utils/ImageOutputDev.cc:44:
In destructor ‘virtual NetPBMWriter::~NetPBMWriter()’,
    inlined from ‘virtual NetPBMWriter::~NetPBMWriter()’ at goo/NetPBMWriter.h:41:31,
    inlined from ‘void ImageOutputDev::writeImage(GfxState*, Object*, Stream*, int, int, GfxImageColorMap*, bool)’ at utils/ImageOutputDev.cc:636:16:
goo/NetPBMWriter.h:41:31: warning: array subscript ‘NetPBMWriter[0]’ is partly outside array bounds of ‘ImgWriter [2]’ [-Warray-bounds]
   41 |     ~NetPBMWriter() override {};
      |                               ^
utils/ImageOutputDev.cc: In member function ‘void ImageOutputDev::writeImage(GfxState*, Object*, Stream*, int, int, GfxImageColorMap*, bool)’:
utils/ImageOutputDev.cc:620:57: note: object of size 16 allocated by ‘operator new’
  620 |             writer = new PNGWriter(PNGWriter::MONOCHROME);
      |                                                         ^
utils/ImageOutputDev.cc:623:51: note: object of size 16 allocated by ‘operator new’
  623 |             writer = new PNGWriter(PNGWriter::GRAY);
      |                                                   ^
utils/ImageOutputDev.cc:627:52: note: object of size 16 allocated by ‘operator new’
  627 |             writer = new PNGWriter(PNGWriter::RGB48);
      |                                                    ^
utils/ImageOutputDev.cc:630:50: note: object of size 16 allocated by ‘operator new’
  630 |             writer = new PNGWriter(PNGWriter::RGB);
      |                                                  ^