vp9-svc: Enable use of second temporal reference for SVC.
vp9-svc: Enable use of second temporal reference for SVC.

When inter-layer is disabled on INTER frames, this will allow
use of a second (longer term) temporal reference for SVC.

Only enabled on highest resolution spatial layer.

Average gains of ~4% on RTC set, speed decrease of about ~2%.

Change-Id: I3c2d415653c448eb7269c828e120fe8bb2ef3f97

webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx