obs-transitions: Fix stinger showing count for update during transition

[exeldro]

Description

Fix stinger showing count for update during transition

Motivation and Context

When a transition is updated while a transition is running the obs_source_remove_active_child was called on an other source than obs_source_add_active_child
the show_refs counter gets to -1, because obs_source_deactivate does only check if it is above 0 for the source it is called on, not for the tree items in hide_tree

How Has This Been Tested?

On windows 11 in combination with move transition having matched items doing a stinger transition

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.

[Lain-B]

I think this looks right. The whole active child thing is annoying and really unsafe. Good catch.