issue with CssRewriteFilter

i have situation where i have a php controller assigned to url http://my.domain.com/cms/assets/core_css.php which returns a a combined css file. When i wanted to rewrite CSS urls for one of the matching the new relative urls, it didn't seem to work.

My asset folder is in the root directory.

Controller:
```
public function action_core_css()
    {
// check for enviroment
        $this->debug = \Fuel::$env == \FUEL::PRODUCTION ? false : true;

        $this->frontendDepend = 'assets'.DS.'frontend-depend'.DS;
        $this->frontendApp = 'assets'.DS.'frontend-app'.DS;

        $this->adminDepend = 'assets'.DS.'admin-depend'.DS;
        $this->adminApp = 'assets'.DS.'admin-app'.DS;

        $ac = new AssetCollection(array(
            new FileAsset($this->adminDepend.'bootstrap'.DS.'css'.DS.'bootstrap.css'),
            new FileAsset($this->adminDepend.'jquery.fileupload'.DS.'css'.DS.'blueimp-gallery.min.css'),
            new FileAsset($this->adminDepend.'jquery.fileupload'.DS.'css'.DS.'jquery.fileupload-ui.css'),
            new FileAsset($this->adminApp.'css'.DS.'warp-uuberadmin-strap.css'),
            new FileAsset($this->adminApp.'css'.DS.'warp-uuberadmin-style.css'),
        ));
        foreach ($ac as $leaf) {
            $leaf->setTargetPath('ubercms/assets/core_css/');
        }
       // $ac->setTargetPath('ubercms/assets/core_css/');

        $am = new AssetManager();
        $am->set('ubercms_core_css', $ac);

        $fm = new FilterManager();
        $fm->set('css_min', new CssMinFilter());
        $fm->set('css_url', new CssRewriteFilter());


        $factory = new AssetFactory('assets');
        $factory->setAssetManager($am);
        $factory->setFilterManager($fm);
        $factory->setDebug($this->debug);
        $factory->addWorker(new CacheBustingWorker());

        // connect asset with filter
        $css = $factory->createAsset(array(
            '@ubercms_core_css',
        ), array(
            'css_url',
            '?css_min' // ignore if dev
        ));

        // save cache
        // assetic takes care of checking if asset is already there
        $content = new AssetCache(
          $css,
          new FilesystemCache('assets'.DS.'cache')
        );

        header('Content-Type: text/css');
        header('Last-Modified: '.gmdate('D, d M Y H:i:s', $content->getLastModified()).' GMT');

        echo $content->dump();
    }
```

created CSS urls where:
```
'../../fonts/glyphicons-halflings-regular.eot
```
instead of:
```
'../../assets/admin-depend/bootstrap/fonts/glyphicons-halflings-regular.eot
```

I looked at the cssRewriteFilter source code and found that i didn't have correct target paths. So i added this to the code:
```
 foreach ($ac as $leaf) {
      $leaf->setTargetPath('ubercms/assets/core_css/');
 }
```
Next i found, that my in the cssRewriteFilter change $sourcePath to $sourceBase in few places because i $sourceBase is the path to the file and the $sourcePath is name of the file plus the extension. I ended up creating a working code. As i only picked up assetic few days ago, i am not sure, if it's my lack of knowledge or is it a bug.