Log Message:
libvpx: update to 1.9.0.

2020-07-29 v1.9.0 "Quacking Duck"
  This release adds support for NV12, a separate library for rate control, as
  well as incremental improvements.

  - Upgrading:
    NV12 support is added to this release.
    A new interface is added for VP9 rate control. The new library libvp9rc.a
    must be linked by applications.
    Googletest is updated to v1.10.0.
    simple_encode.cc is compiled into a new library libsimple_encode.a with
    CONFIG_RATE_CTRL.

  - Enhancement:
    Various changes to improve VP9 SVC, rate control, quality and speed to real
    time encoding.

  - Bug fixes:
    Fix key frame update refresh simulcast flexible svc.
    Fix to disable_16x16part speed feature for real time encoding.
    Fix some signed integer overflows for VP9 rate control.
    Fix initialization of delta_q_uv.
    Fix condition in regulate_q for cyclic refresh.
    Various fixes to dynamic resizing for VP9 SVC.