vp9-svc: On key frame update all reference slots for SVC.
vp9-svc: On key frame update all reference slots for SVC.

Key frame updates the slots corresponding to the 3 references
last/golden/altref, but for SVC where more references buffers
may be in use, especialy for dynamically swithing up/down in layers,
make sure we should update all 8 slots on key frame.

Change-Id: Ifcca12608f420d5bae32b92794a3afe9b6369f77

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