When deciding whether the top-right or bottom-left blocks are
available, we currently always act as if we're using 128x128
superblocks. This means that, when using 64x64 superblocks,
we sometimes conclude that blocks are available when they haven't
been decoded yet!

This typically happens at, for example, mi_row=15, mi_col=16
(for bottom left), where we're at a 64x64 boundary but not
a 128x128 boundary.

This patch fixes the issue by checking based on the signalled
superblock size.

Note: Most of this patch is just threading 'cm' through the
intra prediction process, so that we have access to cm->sb_size
in has_top_right() and has_bottom_left()

Change-Id: I126964c510aafffc870e7cd8b3e64a46abb14b3a