Made changes to allow use of libpng on Windows
Merged in dev-7
Merge remote-tracking branch 'khronos-gltf/dev-7' into libpng
Update CMakeLists to handle libpng better. We now warn if not present and disable functionality. This allows it to work on windows now.
Added include of zlib headers.
Merge pull request #301 from tfili/libpng

Fixed libpng compile issue on Windows
Merge remote-tracking branch 'khronos-gltf/dev-7' into lines