libvpx

[svc] Finalize spatial svc first pass rate control

      1. Save stats for each spatial layer
      2. Add frame buffer management for svc first pass rc
      3. Set default spatial layer to 1
      4. Flush encoder at the end of stream in test app
This only supports spatial svc.
Change-Id: Ia89cfa87bb6394e6c0405b921d86c426d0a0c9ae