frontend: Fix canvas removal causing crashes due to recursive remove calls

[dsaedtler]

Description

Makes some changes to how canvas removal is handled to avoid recursion in the middle of an erase() operation on the vector holding frontend-managed canvases.

Also prevents a similar issue when clearing scene data by replacing .clear() with a dedicated method that also ensures the canvas removal signal is properly fired for each removed canvas.

Motivation and Context

See #12728, which this PR superseedes.

How Has This Been Tested?

Reproduced using the following snipped (just added to the end of the OBSBasicSettings::OBSBasicSettings() constructor):

	obs_video_info ovi;
	obs_get_video_info(&ovi);
	OBSCanvasAutoRelease cv = obs_frontend_add_canvas("test", &ovi, PROGRAM);
	obs_frontend_remove_canvas(cv);
	OBSCanvasAutoRelease cv2 = obs_frontend_add_canvas("test2", &ovi, PROGRAM);

Note that the crash only happens in release builds, not debug.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Tweak (non-breaking change to improve existing 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.