Rather than encoding the loop restoration coefficients at the start of
the frame header, this patch moves them to occur just after certain
top-level superblocks.

You might hope that we could just encode coefficients on top-level
superblocks where the top-left corner of the superblock was also the
top-left corner of the loop restoration tile. Unfortunately, this
can't work with the superres experiment, where the loop restoration
tiles don't necessarily line up with the superblocks. Indeed, in
general there can be multiple different loop restoration coefficients
that apply in a given top-level superblock. This patch defines a
function, av1_loop_restoration_corners_in_sb, which yields the
rectangle [rrow0, rrow1) x [rcol0, rcol1) of loop restoration tiles
whose top left corners lie in this top-level superblock.

The total file size should be unchanged by this patch: the bits have
just been moved from the frame header and spread out among the rest of
the frame.

Change-Id: Icf43b0560964a63dea0d2cd801313f04139188d7