libobs: Add obs_weak_canvas_references_canvas

[exeldro]

Description

Add obs_weak_canvas_references_canvas

Motivation and Context

Make it the same as for sources, outputs, services and encoders.

How Has This Been Tested?

On windows by using it in a plugin

Types of changes

• New feature (non-breaking change which adds functionality)

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.