[libpng15] De-configured build fixes to make a range of deconfiguration options

(such as switching off read or write support) work in more cases.  Also
upgraded pngtest and pngvalid to the libpng 1.6 versions (with some
modifications) which provide more extensive testing.  Replaced pngtest.png
because pngtest writes the ancillary chunks in a different order.
[libpng15] Updated pngvalid.c from libpng16