Auto convert/adapt relative css/less paths to absolute web-root relative paths

I extended AutoMin to convert relative path's in .css and .less files automatically to absolute web-root relative paths.

The code worked in my test's but it would be great if someone could take a look if the code could be optimized.

e.g. I'm not fully satisfied with using a variable to pass the $file_server_path but I didn't know how to pass it as an argument in the callback. In PHP 5.4 this shouldn't be a problem (since I could just use $this-> instead of array($this,..) ) but since Craft requires PHP 5.3 this was the only solution I could come up with.

Also I'm not fully sure if the RegEx is "bullet-proof".

Hope the code is useful.