Switch to libvips' resize, make fastShrinkOnLoad optional (#977)

test/unit/resize.js

@@ -448,4 +448,34 @@ describe('Resize dimensions', function () {
           });
       });
   });
+
+  it('fastShrinkOnLoad: false ensures image is not shifted', function (done) {
+    return sharp(fixtures.inputJpgCenteredImage)
+      .resize(9, 8, {
+        fastShrinkOnLoad: false,
+        centreSampling: true
+      })
+      .png()
+      .toBuffer(function (err, data, info) {
+        if (err) throw err;
+        assert.strictEqual(9, info.width);
+        assert.strictEqual(8, info.height);
+        // higher threshold makes it pass for both jpeg and jpeg-turbo libs
+        fixtures.assertSimilar(fixtures.expected('fast-shrink-on-load-false.png'), data, { threshold: 7 }, done);
+      });
+  });
+
+  it('fastShrinkOnLoad: true (default) might result in shifted image', function (done) {
+    return sharp(fixtures.inputJpgCenteredImage)
+    .resize(9, 8, {
+      centreSampling: true
+    })
+    .png()
+    .toBuffer(function (err, data, info) {
+      if (err) throw err;
+      assert.strictEqual(9, info.width);
+      assert.strictEqual(8, info.height);
+      fixtures.assertSimilar(fixtures.expected('fast-shrink-on-load-true.png'), data, done);
+    });
+  });
 });