Reland "Enable compound prediction for 2+ reference frames in LibaomAv1EncoderV2"

This is a pure reland of commit 08c3fae1991deeeda6880023d0d5b51ea19097a4
The downstream build failure has been fixed.

Original change's description:
> Enable compound prediction for 2+ reference frames in LibaomAv1EncoderV2
>
> Configure AV1E_SET_SVC_REF_FRAME_COMP_PRED in InitEncode() with
> use_comp_pred = {1, 0, 1} (GOLDEN_LAST and ALTREF_LAST). Libaom only
> applies a compound pair when both underlying reference frame aliases are
> marked as referenced in aom_svc_ref_frame_config_t, so this enables
> compound prediction whenever a frame predicts from two or three
> reference buffers (matching LibaomAv1Encoder V1) without per-frame
> reconfiguration.
>
> Add CompoundPredictionUsedForTwoAndThreeReferences in
> LibaomAv1EncoderV2Test, parsing the encoded AV1 OBU frame headers with
> libgav1::ObuParser to verify that reference_mode_select is false for
> single-ref frames and true for both 2-reference (GOLDEN_LAST) and
> 3-reference (ALTREF_LAST) blended frames.
>
> Bug: webrtc:496266459
> Change-Id: Ic9fca08c1ed41588d99a3134211b41a28aff8775
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508140
> Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
> Auto-Submit: Erik Språng <sprang@webrtc.org>
> Commit-Queue: Erik Språng <sprang@webrtc.org>
> Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#48855}

Bug: webrtc:496266459
Change-Id: Ic530b9882b2de469ae2f7305c482f894f475a6a3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508740
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48872}