WebGPURenderer: Fix shadowPositionNode cache

[sunag]

Description

material.shadowPositionNode was losing the custom position reference after the first shadow structure was created, due to caching.

Before	Now

// to reproduce this problem put this code in `webgpu_shadowmap` example
const mesh = new THREE.Mesh( new THREE.BoxGeometry( 7, 4, 7 ), new THREE.MeshStandardMaterial( { color: 0x00ff00 } ) );
mesh.receiveShadow = true;
mesh.castShadow = true;
mesh.position.set( 0, 2, 0 );
scene.add( mesh );

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.13 79	339.13 79	+0 B +0 B
WebGPU	478.67 132.72	478.72 132.72	+50 B +1 B
WebGPU Nodes	478.13 132.6	478.18 132.6	+50 B +2 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	464.59 111.96	464.59 111.96	+0 B +0 B
WebGPU	546.83 148.16	546.88 148.18	+50 B +14 B
WebGPU Nodes	502.71 137.88	502.76 137.89	+50 B +7 B