libvpx

fixed a bug of context overwritten by key frame recoding

The recoding loop save and restore frame coding context for recodes.
However in recoding of key frames, some of the coding context saved
was stale from last encoded inter frame. The save/restore sometimes
overwrites the re-inintialized coding context with saved context
from last frame, resulting in encoder/decoder mismatch

Change-Id: I354ae2f71074d142602d51d06544c05a2462caaf