UI: Add (eRTMP) multitrack video output

[palana]

Note that this PR includes a bunch of merges from currently open PRs and a FIXME commit to help with testing, which will go away as soon as the dependency PRs are merged

Description

This implements multitrack video support for Twitch Enhanced Broadcasting and Amazon IVS Multitrack Video

Motivation and Context

Multitrack video/Enhanced Broadcasting allows sending multiple video tracks/renditions from the users machine, resulting in generally higher quality renditions at lower bitrates compared to single track streaming with server side transcoding, since e.g. losses roundtripping frames through another decode/encode step are removed. Enhanced Broadcasting/multitrack video uses server side configuration to provide easier onboarding for users, and balanced values for the number of renditions and their bitrates.

How Has This Been Tested?

This has been tested in the Twitch Enhanced Broadcasting beta

Types of changes

• New feature (non-breaking change which adds functionality)

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.