Change role of pagination layer to pseudo layer on advanced background

[yhatt]

This PR will change the role of pagination layer on advanced background into a pseudo layer to render pseudo-elements of section.

It will allow to use ::before pseudo-element as a decoration for slide correctly. The pseudo layer will become to always render in advanced backgrounds.

Regular	w/ Split background (previous)	w/ Split background (fixed)
	The gradation can see in each of split area.	The gradation fills whole slide correctly.

/* @theme pseudo-decorated */
section::before {
  content: '';
  display: block;
  position: absolute;
  top: 0;
  right: 0;
  left: 0;
  bottom: 0;
  background: linear-gradient(
    to right,
    rgba(0, 0, 255, 0.1),
    transparent,
    rgba(0, 0, 255, 0.1)
  );
  pointer-events: none;
}

---
theme: pseudo-decorated
---

# Marp

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