vp8 rc: explicit cast to avoid VS build failure
vp8 rc: explicit cast to avoid VS build failure

Change-Id: I6a4daca12b79cf996964661e1af85aa6e258b446

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