#315 png pHYs chunk information is completely wrong

[Billy Biggs]

I had thought that the pHYs chunk was the physical size
of the image. This is completely wrong, it actually
defines pixel aspect ratio exactly. We should fix our
pHYs chunk to give the right information.

See:
http://www.libpng.org/pub/png/libpng-1.2.5-manual.html
http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.pHYs

This is related to the bug we found in the gimp, where
it assumed that pHYs chunks always give the actual
image size in meters:

http://bugzilla.gnome.org/show_bug.cgi?id=123201

So, we both have bugs.