Add alpha channel to VideoFrameBuffer containers

- Add alpha accessors to PlanarYuvBuffer interface, null by defualt.
- Add WrapI420ABuffer() that creates a container which implements these
accessors.
- Show the use via StereoDecoderAdapter.

This CL is the step 2 for adding alpha channel support over the wire in webrtc.
See https://webrtc-review.googlesource.com/c/src/+/7800 for the experimental
CL that gives an idea about how it will come together.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: Id5691cde00088ec811b63d89080d33ad2d6e3939
Reviewed-on: https://webrtc-review.googlesource.com/21130
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Commit-Queue: Emircan Uysaler <emircan@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20635}

modules/video_coding/codecs/stereo/stereo_decoder_adapter.cc

@@ -11,6 +11,7 @@
 #include "modules/video_coding/codecs/stereo/include/stereo_decoder_adapter.h"
 
 #include "api/video/i420_buffer.h"
+#include "api/video/video_frame_buffer.h"
 #include "api/video_codecs/sdp_video_format.h"
 #include "common_video/include/video_frame.h"
 #include "common_video/include/video_frame_buffer.h"
@@ -18,6 +19,11 @@
 #include "rtc_base/keep_ref_until_done.h"
 #include "rtc_base/logging.h"
 
+namespace {
+void KeepBufferRefs(rtc::scoped_refptr<webrtc::VideoFrameBuffer>,
+                    rtc::scoped_refptr<webrtc::VideoFrameBuffer>) {}
+}  // anonymous namespace
+
 namespace webrtc {
 
 class StereoDecoderAdapter::AdapterDecodedImageCallback
@@ -173,11 +179,22 @@ void StereoDecoderAdapter::MergeAlphaImages(
     VideoFrame* alpha_decodedImage,
     const rtc::Optional<int32_t>& alpha_decode_time_ms,
     const rtc::Optional<uint8_t>& alpha_qp) {
-  // TODO(emircan): Merge the output and put in a VideoFrame container that can
-  // transport I420A.
-  decoded_complete_callback_->Decoded(*decodedImage, decode_time_ms, qp);
-  decoded_complete_callback_->Decoded(*alpha_decodedImage, alpha_decode_time_ms,
-                                      alpha_qp);
+  rtc::scoped_refptr<webrtc::I420BufferInterface> yuv_buffer =
+      decodedImage->video_frame_buffer()->ToI420();
+  rtc::scoped_refptr<webrtc::I420BufferInterface> alpha_buffer =
+      alpha_decodedImage->video_frame_buffer()->ToI420();
+  RTC_DCHECK_EQ(yuv_buffer->width(), alpha_buffer->width());
+  RTC_DCHECK_EQ(yuv_buffer->height(), alpha_buffer->height());
+  rtc::scoped_refptr<I420ABufferInterface> merged_buffer = WrapI420ABuffer(
+      yuv_buffer->width(), yuv_buffer->height(), yuv_buffer->DataY(),
+      yuv_buffer->StrideY(), yuv_buffer->DataU(), yuv_buffer->StrideU(),
+      yuv_buffer->DataV(), yuv_buffer->StrideV(), alpha_buffer->DataY(),
+      alpha_buffer->StrideY(),
+      rtc::Bind(&KeepBufferRefs, yuv_buffer, alpha_buffer));
+
+  VideoFrame merged_image(merged_buffer, decodedImage->timestamp(),
+                          0 /* render_time_ms */, decodedImage->rotation());
+  decoded_complete_callback_->Decoded(merged_image, decode_time_ms, qp);
 }
 
 }  // namespace webrtc