Assets path in style files are not replaced

Problem/Motivation

When i display a page with virtual base on a virtual base path, assets defined in css files are not rewrited.

• If have a Virtual base with prefix my_base
• I go to /my_base/node/1
• css files display assets with images, fonts etc with path relative to regular root. example : /core/misc/throbber-inactive.png

Steps to reproduce

Add a url() with an image in one of your css file
Go to virtual_base pages, the image cannot be founded since path is not rewrited with base prefix.