Use imagemagick's compare for the testsuite, and compare with fuzz As changing colorspace might introduce rounding errors, a binary comparison will lead to false failures in the test suite. This patch ceased to use cmp but uses compare with a very low fuzz of 0.5% to see if the image is same, it is used for all tests with convert_colorspace. See also http://www.libpng.org/pub/png/spec/1.2/PNG-Encoders.html for rationale.
