WebGLRenderTarget: Fix copy(). (#23732)

src/renderers/WebGLRenderTarget.js

@@ -78,6 +78,7 @@ class WebGLRenderTarget extends EventDispatcher {
 		this.viewport.copy( source.viewport );
 
 		this.texture = source.texture.clone();
+		this.texture.isRenderTargetTexture = true;
 
 		// ensure image object is not shared, see #20328