If horizontal padding is needed, copy the data to avoid having to try
to write into the input buffer.
If vertical padding is needed, we could get away with only copying
the last MB row (as previously was done), but then one extra row of
pixels are needed for intra pred. Thus always copy all of the picture,
for simplicity, if padding is needed.
Change-Id: I6b456a12956120d64e7c6b86d7b571e159f4566b
4f5ceca078