Implement preview thumbnail generation for 3D and layered textures

[BlueCube3310]

Allows preview generation of 3D and layered textures in the file manager and inspector.

master	PR

TODO:

• Fix errors when retrieving layered textures from RenderingDevice (RenderingDevice: Fix getting cubemap layer data #94656),
• Add a testing project.

Testing project: layered-preview-test.zip

[Calinou]

I wonder if the first layer should always be displayed, or whether we should display an intermediate layer halfway through (e.g. the 128th layer in a 256-layer image).

With some 3D data such as SDFs, the first layer may look like nothing meaningful (e.g. a pure red texture for baked particle collision SDFs).

You could compare this to video streaming websites where the automatically thumbnail is generally picked from the middle of the video, as opposed to its first frame.

[BlueCube3310]

The middle layer is now used for generating previews.

[akien-mga]

Thanks!