Integrate QOI codec completely

* Adds code for encoders and decoders
* Cleans up some obsolete QOI-related code

codecs/qoi/dec/qoi_dec.d.ts

@@ -0,0 +1,7 @@
+export interface QOIModule extends EmscriptenWasm.Module {
+  decode(data: BufferSource): ImageData | null;
+}
+
+declare var moduleFactory: EmscriptenWasm.ModuleFactory<QOIModule>;
+
+export default moduleFactory;