webrtcdsp: echo cancellation issue with stereo signal

Describe your issue

The echo cancellation feature of webrtcdsp breaks the stereo of an audio signal.

Discussed here: https://discourse.gstreamer.org/t/webrtcdsp-stereo-echo-cancellation/739/2

Expected Behavior

Captured stereo should be played back as is by the speakers. gst-launch-1.0 alsasrc ! webrtcdsp echo-cancel=true ! webrtcechoprobe ! audioconvert ! alsasink

Observed Behavior

Only one channel is output. Goes back to normal with echo-cancel=false.

Setup

• Operating System: Ubuntu 22.04
• Device: Computer
• GStreamer Version: 1.23.1 (commit 79ffe4f4)

Steps to reproduce the bug

This pipeline sends two different signals on the left and right channels. Using echo-cancel=true or false, the stereo is degraded.

gst-launch-1.0 \
    audiotestsrc volume=0.1 ! capsfilter caps=audio/x-raw,format=S16LE,rate=8000,channels=1,channel-mask=\(bitmask\)0x1 name=left \
    audiotestsrc freq=1320 volume=0.1 ! capsfilter caps=audio/x-raw,format=S16LE,rate=8000,channels=1,channel-mask=\(bitmask\)0x2 name=right \
    interleave name=i left. ! i. right. ! i. \
        i. ! webrtcdsp echo-cancel=true ! webrtcechoprobe \
    ! audioconvert ! audioresample ! alsasink

How reproducible is the bug?

Systematic

Additional Information

Discussed with @fengalin. The regression seems to have appeared since d5755744