Adding browser's webp encoder (#72)

* Adding WebP (without feature detect in place)

* Adding WebP check

* Remove unused import

src/codecs/browser-webp/encoder.ts

@@ -0,0 +1,23 @@
+import { canvasEncode } from '../../lib/util';
+
+export interface EncodeOptions { quality: number; }
+export interface EncoderState { type: typeof type; options: EncodeOptions; }
+
+export const type = 'browser-webp';
+export const label = 'Browser WebP';
+export const mimeType = 'image/webp';
+export const extension = 'webp';
+export const defaultOptions: EncodeOptions = { quality: 0.5 };
+
+export async function featureTest() {
+  const data = new ImageData(1, 1);
+  const blob = await encode(data, defaultOptions);
+  // According to the spec, the blob should be null if the format isn't supported…
+  if (!blob) return false;
+  // …but Safari falls back to PNG, so we need to check the mime type.
+  return blob.type === mimeType;
+}
+
+export function encode(data: ImageData, { quality }: EncodeOptions) {
+  return canvasEncode(data, mimeType, quality);
+}