Linux: add AudioDeviceLoopbackLinux (OpenAL loopback ADM)

[paterkleomenis]

Summary

Adds a minimal loopback Audio Device Module for Linux that captures system audio output via the OpenAL Soft capture API, by opening the PulseAudio monitor source exposed as an OpenAL capture device.

This is a focused, self-contained PR. It does not include mixing, mic muting, or playback volume control — those are follow-up PRs.

Changes

New files

• webrtc/platform/linux/webrtc_loopback_adm_linux.h

• webrtc/platform/linux/webrtc_loopback_adm_linux.cpp

  Implements AudioDeviceLoopbackLinux (a webrtc::AudioDeviceModule):

  • IsSupported() enumerates ALC_CAPTURE_DEVICE_SPECIFIER to detect whether a PulseAudio monitor source is available.
  • captureLoop() runs on a dedicated std::thread, reads 10 ms stereo-16 chunks via alcCaptureSamples, and delivers them through webrtc::AudioDeviceBuffer to the WebRTC pipeline.
  • Uses the existing desktop-app::external_openal CMake target — no new dependencies.

Modified files

• webrtc/webrtc_create_adm.cpp / .h

  • CreateLoopbackAudioDeviceModule() now instantiates AudioDeviceLoopbackLinux on Linux (mirrors the existing Windows branch).
  • New LoopbackAudioCaptureSupported() function reports whether loopback capture is available on the current platform.

• CMakeLists.txt

  • Adds the two new source files.
  • Adds WEBRTC_LINUX compile definition under elseif (LINUX), matching the existing WEBRTC_WIN / WEBRTC_MAC pattern.

Platform

Linux only. Windows and macOS are unaffected.

Follow-up PRs

• Add MixingAudioDeviceModule and loopback mixing transport #26 — Add MixingAudioDeviceModule and loopback mixing transport (depends on this PR)
• Add mic-only muting and playback volume control to MixingAudioDeviceModule #24 — Add mic-only muting and playback volume control (depends on Add MixingAudioDeviceModule and loopback mixing transport #26)

[paterkleomenis]

Closing in favour of #22, which has been reopened and updated with this exact same clean single-commit implementation. The reviewer preferred to continue with the original PR rather than opening a new one.