Re-use the tile worker threads to pack the bitstream in parallel
on a per-tile basis.  Restricting this to real-time only for now
(further testing is needed to ensure this does not make 2-pass
worse in any case).

BUG=webm:1309

Change-Id: I8a80da7c5089b837d0df79a5c49d5e3022dfc8ec