Convert urls relative to current css files to full

If source CSS contain a relative URL not starting with slash (i.e.
relative to current file), those URL will be converted to full relative
path using MEDIA_URL, which should be the same as if COMPRESS is set to
False. When compress enabled, images will be accessible on the same URL.

compress/utils.py

@@ -86,12 +86,29 @@ def concat(filenames, separator=''):
     """
     Concatenate the files from the list of the ``filenames``, ouput separated with ``separator``.
     """
+    # find relative paths in css:
+    # url definition, any spacing, single or double quotes, no starting slash
+    rel_exp = re.compile(
+        '(url\s*\(\s*[\'"]?\s*)([^/\'"\s]\S+?)(\s*[\'"]?\s*\))',
+        flags=re.IGNORECASE)
     r = ''
     for filename in filenames:
         fd = open(compress_source(filename), 'rb')
-        r += fd.read()
-        r += separator
+        contents = fd.read()
         fd.close()
+        if filename.lower().endswith('.css') and \
+           django_settings.MEDIA_ROOT == settings.COMPRESS_SOURCE:
+            if django_settings.MEDIA_URL.endswith('/'):
+                abspath = os.path.normpath(os.path.dirname(filename))
+            else:
+                abspath = os.path.normpath(os.path.join('/',
+                                                    os.path.dirname(filename)))
+            abspath = django_settings.MEDIA_URL + abspath + '/'
+            fixed = rel_exp.sub('\\1' + abspath + '\\2\\3', contents)
+            r += fixed
+        else:
+            r += contents
+        r += separator
     return r
 
 def save_file(filename, contents):

docs/Configuration.textile

@@ -86,6 +86,32 @@ Network, you can use the optional _prefix_ parameter:
 
 In this example, the template tags will render _http://cdn.example.com/css/one_compressed.css_ in the link tag.  You will need to manually put there after you build as part of your deployment process.
 
+h4. CSS URL not starting with slash
+
+If source CSS contain a relative URL not starting with slash (i.e. relative to current file), those URL will be converted to full relative path using MEDIA_URL. This is done _only when SOURCE_ROOT is the same as MEDIA_ROOT_ (default). This conversion is performed before any filters applied.
+Example:
+<pre><code>
+media/js/fancybox/
+  fancybox.png
+  fancybox-x.png
+  fancybox-y.png
+  jquery.fancybox-1.3.4.css
+  jquery.fancybox-1.3.4.js
+</code></pre>
+jquery.fancybox-1.3.4.css contains:
+<pre><code>
+        background-image: url('fancybox.png');
+        background-image: url('fancybox-x.png');
+        background-image: url('fancybox-y.png');
+</code></pre>
+In resulting CSS it will be rewritten to:
+<pre><code>
+background-image:url(/js/fancybox/fancybox.png);
+background-image:url(/js/fancybox/fancybox-x.png);
+background-image:url(/js/fancybox/fancybox-y.png);
+</code></pre>
+(Assuming MEDIA_URL is '' or '/', with non-empty MEDIA_URL result will be another).
+
 h4. CSS URL Replace Filter
 
 This is a really simple filter for doing Python string replacements on your CSS.  The use case for this is when you have root relative