cherry picked from commit 5d9f79edef2c11b915bdac3a025b59a32082f409

SAO edge filter uses pre-SAO pixel data on the left and top of the ctb, so
this data must be kept available. This was done previously by having 2
copies of the frame, one before and one after SAO.

This commit reduces the storage to just that, instead of the previous whole
frame.

Commit message taken from patch by Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>