Add spatial index to EncodedImage.

Replaces the VP8 simulcast index and VP9 spatial index formely part of
CodecSpecificInfo.

Bug: webrtc:9378
Change-Id: I80eafd63fbdee0a25864338196a690628b4bd3d2
Reviewed-on: https://webrtc-review.googlesource.com/83161
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24485}

modules/video_coding/codecs/vp9/vp9_impl.h

@@ -61,6 +61,7 @@ class VP9EncoderImpl : public VP9Encoder {
   int InitAndSetControlSettings(const VideoCodec* inst);
 
   void PopulateCodecSpecific(CodecSpecificInfo* codec_specific,
+                             absl::optional<int>* spatial_idx,
                              const vpx_codec_cx_pkt& pkt,
                              uint32_t timestamp,
                              bool first_frame_in_picture);