Passing props via layout components

[Rich-Harris]

A minor annoyance with 0.15: if you're using a default layout component, it won't propagate data specified in custom layout components:

<!-- routes/_layout.html -->
<svelte:component
  this={child.component}
  {customThing}
  {child.props}
/>

<script>
  export default () {
    preload() {
      return { customThing: 'potato' };
    }
  };
</script>

<!-- routes/foo/index.html -->
<p>custom thing: {customThing}</p>

You might expect customThing to be passed down to /foo, since it appears to be the child component. But in fact, the child component is the default layout component, and we need to override it like so:

<!-- routes/foo/_layout.html -->
<svelte:component
  this={child.component}
  {customThing}
  {child.props}
/>

One way we could solve this is to remove any intermediate default layout components. In other words, child.component in the top-level layout component would be the routes/foo/index.html component. This would have the added benefit of simplifying Svelte's internal representation of the DOM.