UI: Set NV12 as preferred format for multitrack video encoders

[dsaedtler]

Description

Sets the preferred format for multitrack encoders.

Motivation and Context

Twitch doesn't handle 444 or HDR content properly, and currently stream encoders are already always forced to NV12 (if not HDR):

obs-studio/UI/window-basic-main-outputs.cpp

Lines 1873 to 1883 in 4dd3cf6

	switch (format) {
	case VIDEO_FORMAT_I420:
	case VIDEO_FORMAT_NV12:
	case VIDEO_FORMAT_I010:
	case VIDEO_FORMAT_P010:
	break;
	default:
	obs_encoder_set_preferred_video_format(videoStreaming,
	VIDEO_FORMAT_NV12);
	}

This has been an issue for people that record 444 locally but still stream to Twitch (or other websites for that matter).

See TEB Discord thread: https://discord.com/channels/1171528364000018442/1286889511225393182

How Has This Been Tested?

Streamed to Twitch, confirmed output is now 420.

Types of changes

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

Checklist:

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