Updates the the canvas render target texture's resource. This is where the render target will create or find a canvas object to use. It also callsUpdateResourceImmediate()to clear the render target texture from the deferred rendering list, to stop the texture from being cleared the next frame. From there it will ask the rendering thread to set up the RHI viewport. The canvas is then set up for rendering and then the user's update delegate is called. The canvas is then flushed and the RHI resolves the texture to make it available for rendering.