libvpx

Don't reset sign_bias fields in vp10_setup_past_independence().

The fields are always coded in the frame itself, so there is never any
dependency on past frames. In practice, this fixes sign_bias being
ignored when error_resilient_mode=1.

See issue 1011.

Change-Id: I9d134ef6b445ced4d100fa735ce579855a0fa5af