libobs: Fix canvas private duplicated scenes

[exeldro]

Description

Fix calling obs_scene_duplicate with OBS_SCENE_DUP_PRIVATE_REFS not setting canvas

Motivation and Context

When you use studio mode with duplicate scene you get this warning in the OBS log file:
Attempted to add Scene without specifying a canvas! Using default canvas instead.

How Has This Been Tested?

On windows by enabling studio mode

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.

[Fenrirthviti]

Does this actually have any negative effects outside the log line? I agree it should probably be fixed even if so, but curious what the presenting bug is, if any here to help prioritize it.

[exeldro]

If you have OBS without any plugins that create an extra canvas there is a log line every scene switch when in studio mode. For extra canvas the duplication will get the resolution of the main canvas instead of the resolution the canvas the scene is duplicated from,