Bugfix: Pass class attribute to pseudo section on advanced background

[yhatt]

If you are using Marpit with inline SVG mode (inlineSVG: true), Marpit makes a layer to show section::after pseudo-element for the pagination, that is not affected changing layout by split backgrounds.

When you use paginate directive together with class directive, the created layer has not inherited section's class.

---
paginate: true
class: red
---

<style>
section.red::after {
  color: red;
}
</style>

This page will be colored page number to red in inline SVG mode.

---

![bg left 80%](https://raw.githubusercontent.com/yhatt/marp/master/images/marp.png)

But this page *won't* be colored page number to red in inline SVG mode.

The result of rendering is here (PDF).

This PR will fix this behavior by passing class attribute to <section> on pseudo-layer.