Romain Vimont authored Apr 18, 2019 and Thomas Daede committed Apr 19, 2019

A restoration unit may contain several super-blocks, and may be
"stretched" on borders, even across tile boundaries:
<https://github.com/xiph/rav1e/issues/631#issuecomment-454419152>

In the bitstream, it must be coded only for its first super-block, in
plane order. To do so, a "coded" flag was set the first time, so that
further super-blocks using the same restoration will not "code" it.

But this assumed that all super-blocks associated to a restoration unit
were encoded sequentially in plane order. With parallel tile encoding,
even with proper synchronization (preventing data races), this
introduces a race condition: a "stretched" restoration unit may not be
coded in its first super-block, corrupting the bitstream.

To avoid the problem, expose the restoration unit only for its first
super-block, by returning a Option<&(mut) RestorationUnit>. This also
avoids the need for any synchronization (a restoration unit will never
be retrieved by more than 1 tile).

At frame level, lrf_filter_frame() will still retrieve the correct
restoration unit for each super-block, by calling
restoration_unit_by_stripe().