libpng with CgBI support.

CgBI is Apple's proprietary extension, to the PNG image file format.

This library is an extension to libpng supporting CgBI file format.

The usage is basically identical to that of libpng,

Additionally, color_type returned by png_get_IHDR equals 7 if the opened PNG file is in CgBI format.

Note that it does not byteswap the pixels.