Upgrade AVIF decoding code

- Update to newer APIs.
 - Avoid manual pixel-by-pixel copy in favour of decoding directly to desired format & bit depth.
 - Avoid use-after-free by cloning the Uint8Array Wasm memory view into a JS-owned Uint8Array right away.

codecs/avif_dec/avif_dec.d.ts

@@ -6,7 +6,6 @@ interface RawImage {
 
 interface AVIFModule extends EmscriptenWasm.Module {
   decode(data: BufferSource): RawImage;
-  free_result(): void;
 }
 
 export default function(opts: EmscriptenWasm.ModuleOpts): AVIFModule;