Save and restore 3 rows above and below each stripe, instead of 2.
The extra rows are filled with duplicates of the outermost context
rows.

This should not affect the encoder or decoder output in any way,
as currently these outer rows are not used. But this will enable
later patches to simplify the code and make it a closer match
to the way things are described in the striped-loop-restoration
design document.

Change-Id: I8ae5433e321d6025c6dc1b473330f485f1599340