Example: Update `webgpu_mrt_mask` imports (#28972)

* Example: Update `webgpu_mrt_mask` imports

* WebGPURenderer: Add `webgpu_shadowmap_opacity`

* Revert "WebGPURenderer: Add `webgpu_shadowmap_opacity`"

This reverts commit d08c131dc1f3515a7d97db7e7b62fbe6175bacd4.

examples/webgpu_mrt_mask.html

@@ -16,8 +16,8 @@
 		<script type="importmap">
 			{
 				"imports": {
-					"three": "../src/Three.WebGPU.js",
-					"three/tsl": "../src/Three.WebGPU.js",
+					"three": "../build/three.webgpu.js",
+					"three/tsl": "../build/three.webgpu.js",
 					"three/addons/": "./jsm/"
 				}
 			}