libobs: Only emit core signal handlers for public canvases

[tt2468]

Description

Amends behavior to match other notable object types like sources. Private canvases should never have their events emitted to the core signal handler.

Motivation and Context

This implicitly fixes an obs-websocket crash that can occur when a private canvas with a NULL name is destroyed. For example, in #12442

How Has This Been Tested?

Compiles

Types of changes

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

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.