Previously we were calling aom_extend_frame_borders to generate
extended pixels for use in loop-restoration. This generates quite a
large border, when we only need 3 pixels.

In addition, we were also calling extend_frame, which does the same
thing but with a smaller border, once (in the decoder) or multiple
times (in the encoder) per plane.

This patch tidies all of this up so that we only call extend_frame
once per plane, with the largest border size we need (3px).

It also adds two new #defines. RESTORATION_BORDER is the 3 pixel
border needed to do filtering for a processing
unit. RESTORATION_CTX_VERT is the number of rows saved for each stripe
when doing striped loop restoration.

Change-Id: I2c3ffcc19808f79db195f76d857e2f23da5d8a84