aggregating CSS which contains relative reference

[momo22]

I am aggregating several CSS files. The output file is created in an other directory than the aggregated files. If the aggregated files refers an other file, e.g. an image like this
background: url(../images/bg-searchbx.png) 0 0 no-repeat;
than this reference is not valid anymore.

The aggregator should be aware of this and adapt the reference.