WebGLRenderTarget: clone texture image data when cloning render target (#21719)

* clone rendertarget.texture image data

* rework comment

src/renderers/WebGLRenderTarget.js

@@ -89,6 +89,7 @@ class WebGLRenderTarget extends EventDispatcher {
 		this.viewport.copy( source.viewport );
 
 		this.texture = source.texture.clone();
+		this.texture.image = { ...this.texture.image }; // See #20328.
 
 		this.depthBuffer = source.depthBuffer;
 		this.stencilBuffer = source.stencilBuffer;