Add private maxColourDistance for functional tests

Switch MSE-based tests to use it

Remove experimental MSE-based compare API

test/unit/util.js

@@ -1,6 +1,7 @@
 'use strict';
 
 var assert = require('assert');
+var fixtures = require('../fixtures');
 var sharp = require('../../index');
 
 var defaultConcurrency = sharp.concurrency();