Fix issue with unknown speaker layouts

[pkviet]

Description

When controlling audio through obs, if audio is set to surround sound (ex: 2.1) , sound is not captured if CEF returns a speaker layout which obs treats as unknown. This solves the issue for several CEF speaker layouts. The bug was noticed by @notr1ch .

Motivation and Context

Fix a bug.

How Has This Been Tested?

Tested that stereo audio from a YouTube video is captured when audio is set to 2.1. Such a stereo audio is returned by cef as FL FR FC. Previously, the 3 channels were muted. Now the FL and FR are passed to the 2.1 output.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.