UI: Fix OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED

OBS_FRONTEND_EVENT_PREVIEW_SCENE_CHANGED is supposed to be called
whenever the user changes the current preview scene -- it was almost
never being triggered.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f