This tidies up the code somewhat, factoring the snapshot/restore of
rows and columns in the reference frame into a separate function. The
variable names might also be a little easier to follow.

The generalisation is that av1_predict_intra_block no longer requires
that a rectangular block is exactly 2:1 or 1:2. The new code will also
allow tiling a rectangular transform size (say two 1:2 transforms to
make a 1:4 transform).

Change-Id: Ie03cb01cce4645b49d943533c57a495dddc6e55d