[artwork/transcode] Adjust transcode.c so it can take care of artwork

rescaling, meaning we can do without parallel ffmpeg interfaces.
This also moves artwork rescaling from libswscale to libavfilter, which
seems to fix a problem with PNG rescaling.

src/inputs/file_http.c

@@ -70,7 +70,7 @@ start(struct player_source *ps)
     {
       // We set "wanted" to 1 because the read size doesn't matter to us
       // TODO optimize?
-      ret = transcode(evbuf, 1, ps->input_ctx, &icy_timer);
+      ret = transcode(evbuf, &icy_timer, ps->input_ctx, 1);
       if (ret < 0)
 	break;