Revert "Use absolute error instead of average error for path check"

I would still like to use absolute error in the future because it
would remove the relationship between the number of zeros in the
test pixels and the total error score, but we do not seem to be at
a stage where that is practical yet (bug 725339).

This reverts commit 56be40fbef61fd32672209d38a5e874fea603e82.

Conflicts:
	babl/babl-fish-path.c