(Bug report by Viktor Szaka'ts).

tests/pngunknown-sAPI so it's temporarily marked SKIP.

pngset.c to avoid warnings about dead code.

values that are exactly 2^bit_depth, and work on 16-bit platforms.

in the code the number of elements in an array is calculated using
sizeof.  This generates a compile time constant of type (size_t) which
is then typically assigned to an (unsigned int) or (int). Some versions
of GCC on 64-bit systems warn about the apparent narrowing, even though
the same compiler does apparently generate the correct, in-range,
numeric constant.  This adds appropriate, safe, casts to make the
warnings go away.

"if (x !== 0)"

Folded some long lines.

png_set_* APIs

png_set_unknown_chunks().