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

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

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

2006/09/22  -- fix security problem with too-small error message buffer

2006/09/22  -- fix security problem with too-small gamma-shift array.