Support displaying transparent images in popup

Implementation of the feature request in https://github.com/contao/core/issues/5682
![Bildschirmfoto 2013-04-24 um 11 49 52](https://f.cloud.github.com/assets/481937/419164/5e7d0fe2-acc4-11e2-8818-4ba5f3d88b80.png)