[4.0] Camera2D's preview box uses editor viewport size instead of project settings size

[verdog]

Godot version: 4.0 abf5132
OS/device including version: Manjaro Linux
Issue description:
The preview bounds for Camera2D use the aspect ratio of the editor viewport rather than the actual size of the game defined in settings.

I think this stems from using the size of the viewport that the camera is in, in places such as

godot/scene/2d/camera_2d.cpp

Line 100 in 75e4ccc

Size2 screen_size = viewport->get_visible_rect().size;

and

godot/scene/2d/camera_2d.cpp

Line 284 in 75e4ccc

Size2 screen_size = get_viewport_rect().size;

I was able to hack a partial quick fix by changing those lines to look up the size in the project settings instead of the viewport, but I don't know if that's the best approach. It also led to some issues with the transform, i.e. the aspect ratio of the preview box was correct but its transform was off. This might be because my hacky fix only fixed the get screen size call in the draw notification response and not in get_camera_transform (first example above)

Steps to reproduce:

[akien-mga]

Reopening as #38341 was reverted.