UI: Fix falling back to non-multitrack output after error

[dsaedtler]

Description

When an error occurs that prevents multitrack outputs from starting the user is given an option to continue with the regular stream output, clicking "Yes" on that prompt will crash. This PR fixes that by ensuring the necessary fields are captured by copy rahter than reference, as the nested lambdas would result in captured references not working/being invalid.

Motivation and Context

Reported on TEB Discord: https://discord.com/channels/1171528364000018442/1249911465192849409/1304077629413916744

How Has This Been Tested?

Tested fallback by disabling NVENC.

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.