test: Allow 5 differences before failing a pixel test
test: Allow 5 differences before failing a pixel test

Once in a hundred times or so, anti-aliasing of glyphs, rounded
corners, or similar things produces one or two pixel differences among
tens of thousands of perfectly rendered ones.  They are really hard to
spot in a delta image and not worth doing anything about.

cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit