placeholderImage as a callback about modern-screenshot

[qq15725]

Have supported

<img src="error.webp" style="width: 100px; height: 100px;">

{
  fetch: {
    placeholderImage: cloned => {
      const div = document.createElement('div')
      div.style.background = 'grey'
      div.style.display = 'inline-block'
      for (let i = cloned.style.length - 1; i >= 0; i--) {
        const name = cloned.style.item(i)
        div.style.setProperty(
          name,
          cloned.style.getPropertyValue(name),
          cloned.style.getPropertyPriority(name),
        )
      }
      cloned.parentNode?.insertBefore(div, cloned)
      return 'data:image/png;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7'
    },
  },
}