WebGLRenderer: Don't support double-sided, transmissive objects with `WEBGL_multisampled_render_to_texture`. (#28196)

* WebGLRenderer: Don't support double-sided, transmissive objects with `WEBGL_multisampled_render_to_texture`.

* WebGLRenderer: Add comment.

src/renderers/WebGLRenderer.js

@@ -1480,39 +1480,43 @@ class WebGLRenderer {
 			textures.updateMultisampleRenderTarget( transmissionRenderTarget );
 			textures.updateRenderTargetMipmap( transmissionRenderTarget );
 
-			let renderTargetNeedsUpdate = false;
+			if ( extensions.has( 'WEBGL_multisampled_render_to_texture' ) === false ) { // see #28131
 
-			for ( let i = 0, l = transmissiveObjects.length; i < l; i ++ ) {
+				let renderTargetNeedsUpdate = false;
 
-				const renderItem = transmissiveObjects[ i ];
+				for ( let i = 0, l = transmissiveObjects.length; i < l; i ++ ) {
 
-				const object = renderItem.object;
-				const geometry = renderItem.geometry;
-				const material = renderItem.material;
-				const group = renderItem.group;
+					const renderItem = transmissiveObjects[ i ];
 
-				if ( material.side === DoubleSide && object.layers.test( camera.layers ) ) {
+					const object = renderItem.object;
+					const geometry = renderItem.geometry;
+					const material = renderItem.material;
+					const group = renderItem.group;
 
-					const currentSide = material.side;
+					if ( material.side === DoubleSide && object.layers.test( camera.layers ) ) {
 
-					material.side = BackSide;
-					material.needsUpdate = true;
+						const currentSide = material.side;
 
-					renderObject( object, scene, camera, geometry, material, group );
+						material.side = BackSide;
+						material.needsUpdate = true;
+
+						renderObject( object, scene, camera, geometry, material, group );
 
-					material.side = currentSide;
-					material.needsUpdate = true;
+						material.side = currentSide;
+						material.needsUpdate = true;
 
-					renderTargetNeedsUpdate = true;
+						renderTargetNeedsUpdate = true;
+
+					}
 
 				}
 
-			}
+				if ( renderTargetNeedsUpdate === true ) {
 
-			if ( renderTargetNeedsUpdate === true ) {
+					textures.updateMultisampleRenderTarget( transmissionRenderTarget );
+					textures.updateRenderTargetMipmap( transmissionRenderTarget );
 
-				textures.updateMultisampleRenderTarget( transmissionRenderTarget );
-				textures.updateRenderTargetMipmap( transmissionRenderTarget );
+				}
 
 			}