Changes made to pristine png source by mozilla.org developers.

2007/05/05  -- Zero png_ptr->num_trans on CRC error (bug 374810)

2007/04/19  -- Synced with libpng-1.2.16 tree

2007/03/20  -- Added support for APNG

2006/06/27  -- Synced with libpng-1.2.12 tree

2004/10/07  -- Synced with libpng-1.2.7 tree

2004/10/07  -- add mozpngconf.h (bug 208607)