Implement alpha premultiplication (#507)

* Implement alpha premultiplication

* Add benchmark to resize

* Only display "Premultiply alpha" if it's one of the rust resize types.

* Add comment about division by zero

codecs/resize/benchmark.js

@@ -0,0 +1,41 @@
+// THIS IS NOT A NODE SCRIPT
+// This is a d8 script. Please install jsvu[1] and install v8.
+// Then run `npm run --silent benchmark`.
+// [1]: https://github.com/GoogleChromeLabs/jsvu
+
+self = global = this;
+load('./pkg/resize.js');
+
+async function init() {
+  //  Adjustable constants.
+  const inputDimensions = 2000;
+  const outputDimensions = 1500;
+  const algorithm = 3; // Lanczos
+  const iterations = new Array(100);
+
+  // Constants. Don’t change.
+  const imageByteSize = inputDimensions * inputDimensions * 4;
+  const imageBuffer = new Uint8ClampedArray(imageByteSize);
+
+  const module = await WebAssembly.compile(readbuffer("./pkg/resize_bg.wasm"));
+  await wasm_bindgen(module);
+  [false, true].forEach(premulti => {
+    print(`\npremultiplication: ${premulti}`);
+    print(`==============================`);
+    for (let i = 0; i < 100; i++) {
+      const start = Date.now();
+      wasm_bindgen.resize(imageBuffer, inputDimensions, inputDimensions, outputDimensions, outputDimensions, algorithm, premulti);
+      iterations[i] = Date.now() - start;
+    }
+    const average = iterations.reduce((sum, c) => sum + c) / iterations.length;
+    const stddev = Math.sqrt(
+      iterations
+        .map(i => Math.pow(i - average, 2))
+        .reduce((sum, c) => sum + c) / iterations.length
+    );
+    print(`n = ${iterations.length}`);
+    print(`Average: ${average}`);
+    print(`StdDev: ${stddev}`);
+  });
+}
+init().catch(e => console.error(e, e.stack));