have problem when resolve CSS path used in filter

code like 
    background: none;
    filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src="img/banner.png");

could not be resolved, I highly recommend add the testcase.
And, if I want to integrated our build tools with rework-plugin-url, I could not know which file 
the "url" comes from if the arguments only accept one "url" string.
