Minifying CSS into cache-dir should convert file-internal url()-relative paths properly

[oliveratgithub]

When passing a CSS stylesheet file to fTemplating, and using the Minification option, the minified CSS is stored in the Caches directory. However, when the original CSS stylesheet file has and relies on relative url(../.../file.abc)-paths, those should be adjusted within the minified cached version - to properly point to the original path relatively.