I didn't intend to distribute this but it leaked into the libpng16 source,
so here it is.

only one filter is a candidate for selection. This trades off code size (added
png_setup_*_row_only() functions) for speed.

The fixed size buffer for the file name being processed could have a byte
written beyond the end; a bug where the test was updated without changing the
size of the buffer.  This commit reduces the buffer to the system maximum.

png_getrowbytes could, in theory, return 0; probably only if there is a bug in
libpng but the code now checks.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

In libpng 1.7 pngimage needs to check PNG_WRITE_PNG_SUPPORTED (new in 1.7), not
PNG_WRITE_SUPPORTED because png_write_png can be disabled without disabling
PNG_WRITE_SUPPORTED.  Copied the approach from 1.6 pngcp.c (so this still works
in 1.6 as well.)

Signed-off-by: NJohn Bowler <jbowler@acm.org>

If PNG_PNGCP_TIMING_SUPPORTED is defined maximal resolution CPU time logging of
png_read_png and png_write_png is enabled via the --time command line option.
This is not on by default but is enabled by contrib/conftests/pngcp.dfa
Signed-off-by: NJohn Bowler <jbowler@acm.org>

This adds pngcp to the build together with a pngcp.dfa configuration test; the
test revealed some configuration bugs which are fixed by corrections to the
_SUPPORTED macros.

pngcp builds on all tested configurations and a number of bugs have been fixed
to make this happen relative to the version in libpng 1.7 contrib/examples.
pngcp.dfa will have to be different for 1.7 but pngcp.c should work fine (not
yet tested).  pngcp itself is still missing a usage message; this is a
preliminary version, although since it behaves the same way as 'cp' most unoids
shouldn't have a problem using it correctly.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

code in the PAETH decoder in pngrutil.c. Build with PNG_USE_ABS to use this.