WebGPURenderer: `getTextureLevel` of `background` should multiply `maxMipLevel` (#27820)

* WebGPURenderer: `getTextureLevel` of `background` should multiply `maxMipLevel`

* chore: code clean up

examples/jsm/renderers/common/Background.js

@@ -1,7 +1,7 @@
 import DataMap from './DataMap.js';
 import Color4 from './Color4.js';
 import { Mesh, SphereGeometry, BackSide } from 'three';
-import { vec4, context, normalWorld, backgroundBlurriness, backgroundIntensity, NodeMaterial, modelViewProjection } from '../../nodes/Nodes.js';
+import { vec4, context, normalWorld, backgroundBlurriness, backgroundIntensity, NodeMaterial, modelViewProjection, maxMipLevel } from '../../nodes/Nodes.js';
 
 const _clearColor = new Color4();
 
@@ -53,7 +53,7 @@ class Background extends DataMap {
 				const backgroundMeshNode = context( vec4( backgroundNode ), {
 					// @TODO: Add Texture2D support using node context
 					getUV: () => normalWorld,
-					getTextureLevel: () => backgroundBlurriness
+					getTextureLevel: ( textureNode ) => backgroundBlurriness.mul( maxMipLevel( textureNode ) )
 				} ).mul( backgroundIntensity );
 
 				let viewProj = modelViewProjection();