Add Camera3D preview in Inspector

[timothyqiu]

An alternate solution for https://github.com/godotengine/godot-proposals/issues/1017

https://github.com/godotengine/godot/assets/372476/a77cc860-ecf4-4fa5-b23e-daea809775fd

The main problem to solve is that cameras can't be transformed (inside the 3D viewport) when in preview mode. Turning split viewports on and off for a temporary operation is tedious.

Previewing in the inspector is almost the same as using picture-in-picture. But keeps UI the same as other texture previews.

[JekSun97]

We've been waiting for this PR, and it's finally here! :)

[akien-mga]

I think one use case covered by the picture-in-picture approach as implemented in https://twitter.com/anthonyec/status/1747254280518865349 is that you can pin the preview, and do operations on other nodes in the tree.

With the preview in the inspector, aren't we limited to seeing it only when the Camera3D is selected in the scene tree?

[timothyqiu]

With the preview in the inspector, aren't we limited to seeing it only when the Camera3D is selected in the scene tree?

Yeah, that's the limitation. Feature wise, I think it's the same as spliting the viewport and checking "Preview".