This stores frame contexts alongside a reference frame, and always
uses the frame in reference slot 0 (LAST_FRAME) as the source of
the frame context.

The encoder could then reorder reference frames as to control
which frame context is used, however currently it does not.

Low Latency AWCY result:

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.1438 |  0.4161 |     N/A |   0.0386 | -0.0281 |  0.0453 |     0.2514

https://arewecompressedyet.com/?job=before-frame-context-signaling%402017-06-07T23%3A20%3A49.473Z&job=after-frame-context-signaling%402017-06-07T23%3A21%3A36.117Z

Change-Id: I4f6f9b12cb403573efbf9e5c3077d62f5dedc467