Direct CSS images through Dummy

Reusing the dummy_image function and options to be able to do the following in CSS:

```css
background-image: url('/dummy/dummy.php?image@100x100');
```

All normal image-parameters work here.

Would you be interested in a rewrite of the dummy_image to do away with that ugly /cache/-folder? Dummy could spit out images like the above url, and then resize the image just-in-time, without the need for a cache folder.