This fixes several problems:
- the CSS cache was only updated when either the CSS files for all media
  types or one of the global files was modified
- when only the screen CSS was changed, the screen CSS was regenerated
  but then the cache was printed which either caused duplicated screen
  CSS output when gzip output was disabled or invalid output with gzip
  enabled as first the plain text screen CSS and then the
  gzip-compressed cache file was printed

Now first all CSS files are collected, then the cache is checked once
and only after this the CSS content is collected.