Normalize path relatively to CSS file when using separateCSS option

Hi,

On the project I'm working on we have a bunch of modules that are output in a subdirectory of `config.dir`. 
With the following requirejs options:

```javascript
{
  appDir: "src",
  baseUrl: "foo",
  dir: "dist",

  separateCSS: true,

  modules: [ "bar/baz" ],
  // [ ... ]
}
```

Source `src/foo/bar/baz.css`
```css
#image {
  background-image: url(images/kitten.png);
}
```

Would be compiled into:
```css
#image {
  background-image: url(foo/bar/images/kitten.png);
}
```

which doesn't work since the real base url for that module is `dist/foo/bar`.