fixed a bug with png loading on x86_64 platforms caused by libpng retardedness (it...

fixed a bug with png loading on x86_64 platforms caused by libpng retardedness (it defines png_uint_32 as unsigned long, which is 64bit on such architectures, AND to make it worse this can be modified in pngconf.h on a per platform basis, so we detect the problem on big endian 64bit architectures and fix it there too)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6849 d7cf8633-e32d-0410-b094-e92efae38249