Remove direct use of png_struct and png_info_struct, so that it compiles with libpng >= 1.5.0 (tested with 1.5.4).
Remove direct use of png_struct and png_info_struct, so that it compiles
with libpng >= 1.5.0 (tested with 1.5.4).

oneminot/libcvd,oneminot/libcvd,oneminot/libcvd