Shared backgrounds for 3D model previews

[NicoPennec]

Problem

• On 3D model preview, the uploaded background (skybox) is applied temporarily.

Solution

• Create a new Backgrounds section in the Admin menu to set global HDRi backgrounds.
• Create a new Backgrounds tab in the project settings to manage a list of available backgrounds.
• Add a background selector on 3D model preview player.

[frankrousseau]

Thank you!