Pass URL directly to LessEngine

When ServletContext.getResource() returns a jndi: URL, new File(resource.getPath()) may not be a valid file path. LessEngine.compile() accepts a URL, so just pass the URL through.

A few LessEngine versions after 1.3.0 and including 1.3.3 have rewritten ResourceLoader logic that doesn't handle jndi: URLs. A patch for this has also been provided to that project at https://github.com/asual/lesscss-engine/pull/38

I did not commit an updated jar with these changes.