The superres upscale process only generates cm->height many rows
of pixels, but we were previously using cm->mi_rows*MI_SIZE many.
The latter number are easily available before upscaling, but we
ideally don't want to have to upscale extra rows. So we switch
to only using cm->height many rows inside loop-restoration.

While making this change, I found and fixed an issue with
av1_get_tile_rect() when using horizontal subsampling + horizontal
superres.

Finally, note that this change does introduce an edge case which
isn't obvious from the source code. This has been annotated as a
comment in save_deblock_boundary_lines().

Change-Id: Ide2bd433d45baa0fb1198353d05eb21b9a903157