[vdr] How do I grab images in a background thread?

[Markus Maeder]

As written @
http://www.vdrportal.de/board/thread.php?threadid=30679
I am planning a mosaic plugin. This plugin should draw 3x3 snapshots of
different TV stations and continously refresh the pictures. With the number
1..9 you could switch to the desired TV station.

The snapshots should be taken in a background thread. Therefore I can't 
use the
grab command of the primary device. Is there a simple way to grab images in a
background thread or do I have to decode the mpeg stream in a receiever 
thread?
(Like osdpip does.)