Swap OptiPNG with OxiPNG

This makes building simpler and allows us to potentially use multithreading version in the future.

For now points to a custom fork of OxiPNG that enables WebAssembly support, as PR is still pending review.

src/codecs/oxipng/encoder.ts

@@ -0,0 +1,6 @@
+import { optimise } from '../../../codecs/oxipng/pkg';
+import { EncodeOptions } from './encoder-meta';
+
+export async function compress(data: ArrayBuffer, options: EncodeOptions): Promise<ArrayBuffer> {
+  return optimise(new Uint8Array(data), options.level).buffer;
+}