compiled library size. It never worked properly and as far as we can
tell, no one uses it. The png_set_filter_heuristics() and
png_set_filter_heuristics_fixed() APIs are retained but deprecated
and do nothing.

 simpleover. Also added a comment to png.h pointing out that the pointer must
 be set to NULL!  (simpleover crashes with any slightly complex command lines
 without this fix.)

the Coverity scan without them.

is not supported (John Bowler).  This fixes an error introduced in
libpng-1.6.17beta06.