As suggested in the comments gamma values outside the range currently permitted
by png_set_alpha_mode are useful for HDR data encoding.  These values are
already permitted by png_set_gamma so it is reasonable caution to extend the
png_set_alpha_mode range as HDR imaging systems are starting to emerge.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

The old code incorrectly calculated the output rowbytes when the
application decreased either the number of channels or the bit depth (or
both) in a user transform.  This was safe; libpng overallocated buffer
space (potentially by quite a lot; up to 4 times the amount required)
but, from 1.5.4 on, resulted in a png_error.
Signed-off-by: NJohn Bowler <jbowler@acm.org>

functions. They unnecessarily duplicate png_set_filler() and png_set_add_alpha()
which now work properly with 16-bit images.

input that was made in version 1.6.17beta01, to preserve legacy
behavior even though it was incorrect.  Instead, added new API
png_set_filter_16() and png_set_add_alpha_16() that set a flag to
make png_do_read_filter() interpret the filler bytes properly.

pngrtran.c to suppress clang warnings (Bug report by Viktor Szakats).

  Removed some useless typecasts from contrib/tools/png-fix-itxt.c

(Bug report by Viktor Szaka'ts).

the Coverity scan without them.

Work around more Coverity-scan dead-code warnings.

    needed by png_reciprocal2().
Added #ifdef PNG_16BIT_SUPPORTED/#endif around png_log16bit() and
    png_do_swap().

"if (x !== 0)"