Only affects release builds
Signed-off-by: NJohn Bowler <jbowler@acm.org>

Undefined because (png_byte) << shift is undefined if it changes the signed bit
(because png_byte is promoted to int).  The libpng exported functions
png_get_uint_32 and png_get_uint_16 handle this.

Bug reported by David Drysdale as a result of reports from UBSAN in clang 3.8.

This changes pngvalid to use BE random numbers; this used to produce errors but
these should not be fixed as a result of the previous changes.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

This widens the 'limit' check on the internally calculated error limits in the
'DIGITIZE' case (the code used prior to 1.7 for rgb_to_gray error checks) and it
changes the check to only operate in non-release builds (base build type not RC
or RELEASE.)
Signed-off-by: NJohn Bowler <jbowler@acm.org>

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>