Add experimental overlayWith API

Composites an overlay image with alpha channel into the input image (which
must have alpha channel) using ‘over’ alpha compositing blend mode. This API
requires both images to have the same dimensions.

References:
- http://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
- https://github.com/jcupitt/ruby-vips/issues/28#issuecomment-9014826

See #97.

test/fixtures/index.js

@@ -11,7 +11,7 @@ var getPath = function(filename) {
 
 // Generates a 64-bit-as-binary-string image fingerprint
 // Based on the dHash gradient method - see http://www.hackerfactor.com/blog/index.php?/archives/529-Kind-of-Like-That.html
-var fingerprint = function(image, done) {
+var fingerprint = function(image, callback) {
   sharp(image)
     .greyscale()
     .normalise()
@@ -21,7 +21,7 @@ var fingerprint = function(image, done) {
     .raw()
     .toBuffer(function(err, data) {
       if (err) {
-        done(err);
+        callback(err);
       } else {
         var fingerprint = '';
         for (var col = 0; col < 8; col++) {
@@ -32,7 +32,7 @@ var fingerprint = function(image, done) {
             fingerprint = fingerprint + (left < right ? '1' : '0');
           }
         }
-        done(null, fingerprint);
+        callback(null, fingerprint);
       }
     });
 };
@@ -52,6 +52,11 @@ module.exports = {
   inputPngWithTransparency: getPath('blackbug.png'), // public domain
   inputPngWithGreyAlpha: getPath('grey-8bit-alpha.png'),
   inputPngWithOneColor: getPath('2x2_fdcce6.png'),
+  inputPngOverlayLayer0: getPath('alpha-layer-0-background.png'),
+  inputPngOverlayLayer1: getPath('alpha-layer-1-fill.png'),
+  inputPngOverlayLayer2: getPath('alpha-layer-2-ink.png'),
+  inputPngOverlayLayer1LowAlpha: getPath('alpha-layer-1-fill-low-alpha.png'),
+  inputPngOverlayLayer2LowAlpha: getPath('alpha-layer-2-ink-low-alpha.png'),
 
   inputWebP: getPath('4.webp'), // http://www.gstatic.com/webp/gallery/4.webp
   inputTiff: getPath('G31D.TIF'), // http://www.fileformat.info/format/tiff/sample/e6c9a6e5253348f4aef6d17b534360ab/index.htm
@@ -75,19 +80,46 @@ module.exports = {
   },
 
   // Verify similarity of expected vs actual images via fingerprint
-  assertSimilar: function(expectedImage, actualImage, done) {
+  // Specify distance threshold using `options={threshold: 42}`, default
+  // `threshold` is 5;
+  assertSimilar: function(expectedImage, actualImage, options, callback) {
+    if (typeof options === 'function') {
+      callback = options;
+      options = {};
+    }
+
+    if (typeof options === 'undefined' && options === null) {
+      options = {};
+    }
+
+    if (options.threshold === null || typeof options.threshold === 'undefined') {
+      options.threshold = 5; // ~7% threshold
+    }
+
+    if (typeof options.threshold !== 'number') {
+      throw new TypeError('`options.threshold` must be a number');
+    }
+
+    if (typeof callback !== 'function') {
+      throw new TypeError('`callback` must be a function');
+    }
+
     fingerprint(expectedImage, function(err, expectedFingerprint) {
-      if (err) throw err;
+      if (err) return callback(err);
       fingerprint(actualImage, function(err, actualFingerprint) {
-        if (err) throw err;
+        if (err) return callback(err);
         var distance = 0;
         for (var i = 0; i < 64; i++) {
           if (expectedFingerprint[i] !== actualFingerprint[i]) {
             distance++;
           }
         }
-        assert.strictEqual(true, distance <= 5);  // ~7% threshold
-        done();
+
+        if (distance > options.threshold) {
+          return callback(new Error('Maximum similarity distance: ' + options.threshold + '. Actual: ' + distance));
+        }
+
+        callback();
       });
     });
   }