Make expand_css a global setting

Currently 'expand_css' is a theme function var, but you can't actually set it, anywhere. The only way to change it globally:

/**
 * Implements hook_theme_registry_alter().
 */
function YOURMOD_theme_registry_alter(&$registry) {
  $registry['print']['variables']['expand_css'] = TRUE;
}

which is weird. I think this should be a global setting, like Access images via local file access (they're even slighly related).

Maybe a patch later.