frontend: Fix setting the T-bar position through the API

[djordje-mancic]

Description

Fixes the obs_frontend_set_tbar_position function and makes it actually set the position (value) of the T-bar slider instead of invoking the TBarChanged method directly.

Motivation and Context

This fix is required as, without it, it is impossible to perform T-bar transitions through the API.

This is because the TBarReleased method, which is responsible for finishing the transition from one scene to another, checks the T-bar position, which never gets set by this function.

Fixing this function would make it possible to implement support for physical T-bars on video switcher boards through plugins.

Resolves issue 11372.

How Has This Been Tested?

I've tested this fix with a plugin that lets me control the T-bar position by moving a physical T-bar.

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.

[djordje-mancic]

dup of #13116

I'm not sure it's fair to call my PR a duplicate and then force-push very similar code to your repo afterwards, but I'm fine with any one's PR getting merged as long as it fixes this bug in OBS.

[psyirius]

I'm not sure it's fair to call my PR a duplicate and then force-push very similar code to your repo afterwards, but I'm fine with any one's PR getting merged as long as it fixes this bug in OBS.

My bad.
FTR: I actually simplified my pr before this pr alive. Just haven't committed and pushed the change.

Just checked that the simplified one is similar to what you have done.
Didn't realize it'd make me look bad. lol

Yeah, yours too look neat!

Closing #13116 in favor of this.

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