obs-ffmpeg: Fix 7.1 ALAC encoding

[pkviet]

Description

Commit [1] added ALAC & PCM support. But 7.1 ALAC encoding fails. This fixes the issue by assigning the correct 7.1 layout supported by FFmpeg ALAC encoder (7.1(wide)).

[1] https://github.com/obsproject/obs-studio/commit/3ae98511d09f1ebaf5c9cc005a285efd1a26aeff

Motivation and Context

Fix a bug. I came upon this bug while reviewing Rodney's hybrid mp4 muxer. https://github.com/obsproject/obs-studio/pull/10608 The bug is due to the fact that FFmpeg ALAC encoder uses 7.1(wide) layout which changes the back speakers to side ones.

How Has This Been Tested?

There's no error popup anymore, the recording works fine.

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.