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/unit/overlay.js

@@ -0,0 +1,61 @@
+'use strict';
+
+var fixtures = require('../fixtures');
+var fs = require('fs');
+var sharp = require('../../index');
+
+sharp.cache(0);
+
+// Main
+describe('Overlays', function() {
+  it('Overlay transparent PNG on solid background', function(done) {
+    sharp(fixtures.inputPngOverlayLayer0)
+      .overlayWith(fixtures.inputPngOverlayLayer1)
+      .toBuffer(function (error, data, info) {
+        if (error) return done(error);
+
+        fixtures.assertSimilar(fixtures.expected('alpha-layer-01.png'), data, {threshold: 0}, done);
+      });
+  });
+
+  it('Composite three transparent PNGs into one', function(done) {
+    sharp(fixtures.inputPngOverlayLayer0)
+      .overlayWith(fixtures.inputPngOverlayLayer1)
+      .toBuffer(function (error, data, info) {
+        if (error) return done(error);
+
+        sharp(data)
+          .overlayWith(fixtures.inputPngOverlayLayer2)
+          .toBuffer(function (error, data, info) {
+            if (error) return done(error);
+
+            fixtures.assertSimilar(fixtures.expected('alpha-layer-012.png'), data, {threshold: 0}, done);
+          });
+      });
+  });
+
+  // This tests that alpha channel unpremultiplication is correct:
+  it('Composite three low-alpha transparent PNGs into one', function(done) {
+    sharp(fixtures.inputPngOverlayLayer1LowAlpha)
+      .overlayWith(fixtures.inputPngOverlayLayer2LowAlpha)
+      .toBuffer(function (error, data, info) {
+        if (error) return done(error);
+
+        fixtures.assertSimilar(fixtures.expected('alpha-layer-012-low-alpha.png'), data, {threshold: 0}, done);
+      });
+  });
+
+  // This tests that alpha channel unpremultiplication is correct:
+  it('Composite transparent PNG onto JPEG', function(done) {
+    sharp(fixtures.inputJpg)
+      .overlayWith(fixtures.inputPngOverlayLayer1)
+      .toBuffer(function (error, data, info) {
+        if (error.message !== 'Input image must have an alpha channel') {
+          return done(new Error('Unexpected error: ' + error.message));
+        }
+
+        done();
+      });
+  });
+
+});