Right now, flatten_css_imports requires rewrite_css, and we're in the process 
of changing it to *imply* rewrite_css (unless rewrite_css is explicitly 
forbidden, in which case flattening will be forbidden too).

We could imagine having a "flatten only" mode for css rewriting, where we don't 
rewrite images (even if image rewriting is on) and we keep the minified css iff 
some flattening occurred.  This would avoid some "explicitly forbidden" checks 
in the configuration and possibly enable a configuration that'd actually be 
useful in practice.