libvpx

[spatial svc]Another workaround to avoid using prev_mi

We encode a empty invisible frame in front of the base layer frame to
avoid using prev_mi. Since there's a restriction for reference frame
scaling factor, we have to make it smaller and smaller gradually until
its size is 16x16.

Change remerged.

Change-Id: I9efab38bba7da86e056fbe8f663e711c5df38449