Reland "Wrap Alpha and YUV frame into one EncodedImage for transmission"

This reverts commit d756fd06fed1b6c65dcb263cbd8f00ca23d72f3b.

Original change's description:
> Revert "Wrap Alpha and YUV frame into one EncodedImage for transmission"
>
> This reverts commit 5670c86aeccc9bc1191725431de7998d21b73c07.
>
> Reason for revert: Breaks downstream build. Need to add "#include <cstring>" to stereo_encoder_adapter.cc to use std::memcpy.
>
> Original change's description:
> > Wrap Alpha and YUV frame into one EncodedImage for transmission
> >
> > With alpha channel, we observe the artifacts on the receiver side, and
> > the reason is that when YUV channel has a key frame, it gives frame_buffer2
> > a chance to drop some previous frames. Then it is possible that some alpha
> > frames got dropped, which break the alpha frame dependence chain.
> >
> > In this CL, we pack the YUV frame and alpha encoded frame together as one
> > entity to solve the issue.
> >
> > Bug: webrtc:8773
> > Change-Id: Ibe746a46cb41fd92b399a7069e1d89f02f292af7
> > Reviewed-on: https://webrtc-review.googlesource.com/38481
> > Commit-Queue: Qiang Chen <qiangchen@chromium.org>
> > Reviewed-by: Emircan Uysaler <emircan@webrtc.org>
> > Cr-Commit-Position: refs/heads/master@{#21737}
>
> TBR=qiangchen@chromium.org,emircan@webrtc.org
>
> Change-Id: I11eff814ce093bf6db327ebcd21b1b71a1929849
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: webrtc:8773
> Reviewed-on: https://webrtc-review.googlesource.com/43260
> Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
> Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#21739}

TBR=deadbeef@webrtc.org,qiangchen@chromium.org,emircan@webrtc.org

Change-Id: I0d64b7e7a62e4f35aa012270d3826a23b3fb2337
Bug: webrtc:8773
Reviewed-on: https://webrtc-review.googlesource.com/43440
Commit-Queue: Qiang Chen <qiangchen@chromium.org>
Reviewed-by: Qiang Chen <qiangchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#21749}
diff --git a/modules/video_coding/BUILD.gn b/modules/video_coding/BUILD.gn
index 85edeeb..d493e4e 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -292,8 +292,10 @@
 
 rtc_static_library("webrtc_stereo") {
   sources = [
+    "codecs/stereo/include/multiplex_encoded_image_packer.h",
     "codecs/stereo/include/stereo_decoder_adapter.h",
     "codecs/stereo/include/stereo_encoder_adapter.h",
+    "codecs/stereo/multiplex_encoded_image_packer.cc",
     "codecs/stereo/stereo_decoder_adapter.cc",
     "codecs/stereo/stereo_encoder_adapter.cc",
   ]
@@ -314,6 +316,7 @@
     "../../common_video:common_video",
     "../../rtc_base:rtc_base",
     "../../system_wrappers",
+    "../rtp_rtcp:rtp_rtcp_format",
   ]
 }