PBR: Fix realtime filtering for refraction

[Popov72]

See https://forum.babylonjs.com/t/es6-import-of-babylonjs-core-ignoring-side-effects-required-for-khr-materials-transmission/38198

Realtime filtering can only work if the refraction texture is a cube (the radiance function is expecting a CubeSampler). So, we should disable realtime filtering when the refraction texture is a 2D texture.

[bjsplat]

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[bjsplat]

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