Write png with compression level 9 is pretty slow

[rtlprmft]

I have realized that writing png files with the highest level of compression is pretty slow (actually almost 5 times slower than when I compress an raw image for example with ImageMagick). I believe that the reason are unnecessary (and maybe even unwanted) conversions in libgd. I see the beauty of libgd as you need to run with only one libarary, but as libgd already links with libpng anyways, it will be available. I would volunteer to implement the possibility to write images directly with libpng as I believe it will be significantly faster.