Removed usage of the opaque png info_ptr struct.

The info_ptr structure should not be used, this has been in the png
documentation since at least July 2000.

Hopefully this mantains the same functionality, but in a way that will
compile with the newest versions of libpng.
Fix previous commit so that it doesn't break the build with libpng v12.0
Use png_bytep instead of png_bytepp for the iCCP profile

As suggested by Andrew Ruder in https://github.com/mono/libgdiplus/pull/3
to avoid storing the profile pointer in an undefined memory location.
Merge pull request #8 from knocte/fix_build_with_libpng15

Fix build with libpng15