Look for autoload.php in alternative/installed location

When BundleFu is installed via Composer, autoload.php is in
../../../autoload.php.
CssUrlRewriteFilter: handle "src" in AlphaImageLoader

The DXImageTransform.Microsoft.AlphaImageLoader filter is often used to
simulate alpha transparency with Internet Explorer. The `src` attribute
needs to get rewritten just like normal `url()` instances.