png: Correct endianness for big-endian machines

png: Correct endianness for big-endian machines

libpng wants to receive samples in either RGB or RGBA order, whether
each sample is big-endian or not. This resolves test failures in
testsuite/gdk/memorytexture.c (and a lot of reftests) on s390x, and
probably the PowerPC family too.

Modifying the test to show the color in use and write out the PNG bytes
to a file, and running the memorytexture test on s390x, produces a PNG
that loads with the correct color values in GIMP (on an x86_64 machine),
which seems like evidence that this is the correct change and not just
compensating errors.

Resolves: https://gitlab.gnome.org/GNOME/gtk/-/issues/4616
Signed-off-by: Simon McVittie <smcv@debian.org>