Allow use of nested render arrays in preview content

[aleksip]

E.g. in this case the pattern shila_image is rendered, but the render array in shila_image's image field is not rendered.

shila_card:
  label: Card
  description: A card component
  fields:
    image:
      type: render
      label: Image
      description: Card image. Optional.
      preview:
        type: pattern
        id: shila_image
        fields:
          image:
            '#type': image
            '#uri': http://lorempixel.com/300/200/nature/
            '#width': 300
            '#attributes': {}