CoverArtBox: preserve cover art aspect ratio on drawing


Cover is now placed correctly over the cover shadow, aspect ratio
is preserved.
![Capture du 2013-01-11 18:21:15](https://f.cloud.github.com/assets/151042/60558/7cf68946-5c13-11e2-8f3e-8286a3009b4d.png)
![Capture du 2013-01-11 18:20:06](https://f.cloud.github.com/assets/151042/60560/99d5d8a0-5c13-11e2-9948-446cb6701fe4.png)