Thoggen DVD Ripper

Hi,
as discussed on IRC there seems to be a bug somewhere.

When opening the cropping preview dialog (no matter if this is a DVD, DVD
image, copied DVD contents) it only shows up after an always changing
amount of time between 0 and ~60 seconds.

This is with latest GStreamer-everything and a longer hang happens fairly
often for me.

While hanging one can see parts of the dialog but the main loop seems to be
blocked and 100% CPU is used.