Once RestorationState will be stored into FrameState, it will not be
possible to borrow a FrameState while its RestorationState is mutably
borrowed.

In particular, this prevents passing a FrameState to a non-const
RestorationState method.

For lrf_optimize_superblock(), which is not implemented yet, just remove
the FrameState parameter.

For lrf_filter_frame(), only borrow the "rec" field instead of the whole
FrameState.