libobs: Fix global transition source signals

[Penwy]

Description

This ensures the global transition source signals, source_transition_start, source_transition_stop and source_transition_video_stop actually get fired properly on the global signal handler.

Motivation and Context

Due to the use of obs_source_dosignal and to all transitions being created as private sources, those global signals were not fired, despite being implemented, and documented. This felt like an oversight.

How Has This Been Tested?

Ubuntu 22.04, verified all signals got properly fired.

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.