Fix --preload-file on image files on Opera.

Do not attempt browser image decoding if the browser does not have the window.URL or window.webkitURL objects. Fix the warning print to appear in that case on Opera/9.80 (Windows NT 6.1; WOW64) Presto/2.12.388 Version/12.16.

This allows loading image files without attempting to use browser codecs on Opera, e.g. using stb_image or similar custom C++ code.