WebGPURenderer: Added double-side transmission 1/2 (#29705)

* added double-side transmission

* Update webgpu_materials_transmission.jpg

src/nodes/functions/PhysicalLightingModel.js

@@ -20,6 +20,7 @@ import { screenSize } from '../display/ScreenNode.js';
 import { viewportMipTexture } from '../display/ViewportTextureNode.js';
 import { textureBicubic } from '../accessors/TextureBicubic.js';
 import { Loop } from '../utils/LoopNode.js';
+import { BackSide } from '../../constants.js';
 
 //
 // Transmission
@@ -67,11 +68,14 @@ const applyIorToRoughness = /*@__PURE__*/ Fn( ( [ roughness, ior ] ) => {
 	]
 } );
 
-const singleViewportMipTexture = /*@__PURE__*/ viewportMipTexture();
+const viewportBackSideTexture = /*@__PURE__*/ viewportMipTexture();
+const viewportFrontSideTexture = /*@__PURE__*/ viewportMipTexture();
 
-const getTransmissionSample = /*@__PURE__*/ Fn( ( [ fragCoord, roughness, ior ] ) => {
+const getTransmissionSample = /*@__PURE__*/ Fn( ( [ fragCoord, roughness, ior ], { material } ) => {
 
-	const transmissionSample = singleViewportMipTexture.uv( fragCoord );
+	const vTexture = material.side == BackSide ? viewportBackSideTexture : viewportFrontSideTexture;
+
+	const transmissionSample = vTexture.uv( fragCoord );
 	//const transmissionSample = viewportMipTexture( fragCoord );
 
 	const lod = log2( screenSize.x ).mul( applyIorToRoughness( roughness, ior ) );