Sandbox texture list grows by 2 each time an environment is selected from the dropdown

[bghgary]

Repro

• Open the sandbox with a model (e.g., MetalRoughSpheres.gltf)
• Note that before selecting environments, the texture list looks like this in the inspector:
• Select a few different environments (e.g., Default, Plaza, Studio).
• Expected result: The old environment textures to be disposed
• Current result: The old environment textures stick around.