libvpx

[spatial svc]Multiple frame context feature

We can use one frame context for each layer so that we don't have
to reset the probs every frame. But we can't use prev_mi since we
may drop enhancement layers. So we have to generate a non vp9
compatible bitstream and modify it in the player.
1. We need to code all frames as invisible frame to let prev_mi
   not to be used. But in the bitstream we need to code the
   show_frame flag to 1 so that the publisher will know it's
   supposed to be a visible frame.
2. In the player we need to change the show_frame flag to 0 for
   all frames. Then add an one byte frame into the super frame
   to tell the decoder which layer we want to show.
Change-Id: I75b7304cf31f0ab952f043e33c034495e88f01f3