Parse CSS files for url() to correct for paths

[petemounce]

Current limitation - CSS files must at the same depth as that of the include-combiner action call, because url() within CSS files should be relative to the CSS file.

Don't want to force absolute paths in CSS files.
Don't want to break background-images within CSS files, if they come from a different depth.
Don't want to have one CSS file for those parts that use url() references.

So, parse the CSS files when they're registered and change the url() references to the correct depth.