message: When reconfiguring VP9 restore previous input pixel format
  Otherwise if the pixel format is not I420, the image buffer will
  need to be reallocated on each reconfiguration.
  Cr-Commit-Position: refs/heads/master@{#32281}
diff: @@ -490,6 +490,9 @@ int VP9EncoderImpl::InitEncode(const VideoCodec* inst,
  return WEBRTC_VIDEO_CODEC_ERR_PARAMETER;
  }
  + absl::optional<vpx_img_fmt_t> previous_img_fmt =
  + raw_ ? absl::make_optional<vpx_img_fmt_t>(raw_->fmt) : absl::nullopt;
  +
  int ret_val = Release();
  if (ret_val < 0) {
  return ret_val;
  @@ -530,7 +533,7 @@ int VP9EncoderImpl::InitEncode(const VideoCodec* inst,
  unsigned int bits_for_storage = 8;
  switch (profile_) {
  case VP9Profile::kProfile0:
  - img_fmt = VPX_IMG_FMT_I420;
  + img_fmt = previous_img_fmt.value_or(VPX_IMG_FMT_I420);
  bits_for_storage = 8;
  config_->g_bit_depth = VPX_BITS_8;
  config_->g_profile = 0;
  