BUG: theme switching preview

[Richard Hult <rhult hem2 passagen se>]

This exposes a bug somewhere:

1. Bring up control-center, themes
2. Switch to a theme that uses another engine
3. Switch back to the first theme
4. Preview does no longer work

Other things that does not work entirely: the scrollbars don't get
drawn sometimes in the preview. This is harder to reproduce, but it
happens every now and then if you play around with the themes.

Richard.