bash could not parse (Bug report by Nelson Beebe).

In libpng 1.6 zlib initialization was changed to use the window size in the zlib
stream, not a fixed value.  This causes some invalid images, ones where CINFO is
too large, to display 'correctly' if the rest of the data is valid.  This
provides a work-round for zlib versions where the error arises (ones that
support the API change to use the window size in the stream).
Signed-off-by: NJohn Bowler <jbowler@acm.org>

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

Tests backported from the multi-version compatible ones in libpng 1.7
Signed-off-by: NJohn Bowler <jbowler@acm.org>

not info_ptr.

```
pngread.c:2841:10: warning: 'break' will never be executed [-Wunreachable-code-break]
         break;
         ^~~~~
```

png_handle_pCAL() (Bug report by John Regehr).

    are derived from previous versions.
Removed much of the long list of previous versions from png.h and
    libpng.3.

in png_set_cHRM_XYZ_fixed().