Add private maxColourDistance for functional tests

Switch MSE-based tests to use it

Remove experimental MSE-based compare API

test/unit/fixtures.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var assert = require('assert');
+var fixtures = require('../fixtures');
+
+describe('Test fixtures', function() {
+  describe('assertMaxColourDistance', function() {
+    it('should throw an Error when images have a different number of channels', function() {
+      assert.throws(function() {
+        fixtures.assertMaxColourDistance(fixtures.inputPngOverlayLayer1, fixtures.inputJpg);
+      });
+    });
+    it('should throw an Error when images have different dimensions', function() {
+      assert.throws(function() {
+        fixtures.assertMaxColourDistance(fixtures.inputJpg, fixtures.inputJpgWithExif);
+      });
+    });
+    it('should accept a zero threshold when comparing an image to itself', function() {
+      var image = fixtures.inputPngOverlayLayer0;
+      fixtures.assertMaxColourDistance(image, image, 0);
+    });
+    it('should accept a numeric threshold for two different images', function() {
+      fixtures.assertMaxColourDistance(fixtures.inputPngOverlayLayer0, fixtures.inputPngOverlayLayer1, 100);
+    });
+  });
+});