preserve API compatibility, the new defines all default to "extern"
(requested by Jan Nijtmans).

  Removed some useless typecasts from contrib/tools/png-fix-itxt.c

handling in contrib/libtests/pngstest.c; possible overflow of
unsigned char in contrib/tools/png-fix-itxt.c). To use the "secure"
file handling, define PNG_USE_MKSTEMP, otherwise "tmpfile()" will
continue to be used.

pngstest.c, and pngimage.c. Most seem harmless, but png-fix-itxt
would only work with iTXt chunks with length 255 or less.

variables in the Try macro needs to be volatile to prevent value
being lost over the setjmp.

function has apparently never been used. It was implemented
to support back-door modification of png_struct in libpng-1.4.x
but was apparently never tested (because it does nothing and cannot
do anything).

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.

pngwutil.c, introduced in libpng-0.95, March 1997.

contrib/tools/pngfix.c to avoid confusion with the libpng private macros.

sub_row, up_row, avg_row, and paeth_row into try_row and tst_row.

PNG_DEBUG builds.

type and can be defined on the command line, allowing testing in beta builds. Back
ported from libpng17.

(x + (x>>16) + 32769) works as well as the previous form and avoids one shift.

 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.)

alpha compositing of multiple images, using the "simplified API"