Premultiply alpha channel to avoid dark artifacts during tranformation

Add `Sharp.compare(file1, file2, callback)` function for comparing images
using mean squared error (MSE). This is useful for unit tests.

See:
- https://github.com/jcupitt/libvips/issues/291
- http://entropymine.com/imageworsener/resizealpha/

test/unit/alpha.js

@@ -1,9 +1,8 @@
 'use strict';
 
 var assert = require('assert');
-
-var sharp = require('../../index');
 var fixtures = require('../fixtures');
+var sharp = require('../../index');
 
 sharp.cache(0);
 
@@ -76,4 +75,30 @@ describe('Alpha transparency', function() {
       });
   });
 
+  it('Enlargement with non-nearest neighbor interpolation shouldn’t cause dark edges', function(done) {
+    var BASE_NAME = 'alpha-premultiply-enlargement-2048x1536-paper.png';
+    var actual = fixtures.path('output.' + BASE_NAME);
+    var expected = fixtures.expected(BASE_NAME);
+    sharp(fixtures.inputPngAlphaPremultiplicationSmall)
+      .resize(2048, 1536)
+      .interpolateWith('bicubic')
+      .toFile(actual, function(err) {
+        if (err) throw err;
+        fixtures.assertEqual(actual, expected, done);
+      });
+  });
+
+  it('Reduction with non-nearest neighbor interpolation shouldn’t cause dark edges', function(done) {
+    var BASE_NAME = 'alpha-premultiply-reduction-1024x768-paper.png';
+    var actual = fixtures.path('output.' + BASE_NAME);
+    var expected = fixtures.expected(BASE_NAME);
+    sharp(fixtures.inputPngAlphaPremultiplicationLarge)
+      .resize(1024, 768)
+      .interpolateWith('bicubic')
+      .toFile(actual, function(err) {
+        if (err) throw err;
+        fixtures.assertEqual(actual, expected, done);
+      });
+  });
+
 });