Fixes to pngvalid for various reduced build configurations (eliminate unused
statics) and a fix for the case in rgb_to_gray when the digitize option reduces
graylo to 0, producing a large error.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

This 'fixes' the current problem in the BE tests by not testing it, making the
BE code the same as the LE version.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

One fix is required for libpng16 LE, more for libpng16 BE
Signed-off-by: NJohn Bowler <jbowler@acm.org>

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).