This implements an API and provides a number of assist macros to allow an
application which uses the simplified API write to bypass stdio and write
directly to memory.

It also includes some warnings (png.h) and some check code to detect *possible*
overflow in the ROW_STRIDE and simplified image SIZE macros.  This disallows
image width/height/format that *might* overflow.  A quiet API change that limits
in-memory image size (uncompressed) to less that 4GByte and image row size
(stride) to less than 2GByte.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

pngunknown.c calls png_init_io (always), skip the test if there is no stdio.h
support.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

"tmpfile()" implementation in contrib/libtests/pngstest.c

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>