frontend: Switch off deleted scenes immediately

[Warchamp7]

Description

Companion PR to #12882. When a scene has been removed and it was the previously active scene, switch immediately off of it.

Motivation and Context

Fixes #12880 on the frontend side whereas #12882 fixes it on the libobs side. Both solutions are good: libobs should immediately cease using any source marked as removed and the frontend should avoid putting libobs in a state where it needs to.

How Has This Been Tested?

Deleted multiple scenes that contained Audio Input/Output captures. Performed transitions after deleting the scene and confirmed no "ghost" transitions happened caused by the scene still being held by the current transition.

Types of changes

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

• Tweak (non-breaking change to improve existing 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.