在柱状图中无法设定slider的padding

* **G2Plot Version**:2.3.18
* **Platform**:g2plot 2.x
* **Mini Showcase(like screenshots)**:
* **CodePen Link**:https://codesandbox.io/s/festive-maxwell-2irjk?from-embed=&file=/index.ts

在柱状图中引入slider后，在不设定外层padding的情况下，设定slider的padding能够生效；在设定了外层的padding后，再去设定slider的padding出现不生效的情况，导致slider都与横轴重叠在一起了。有没有什么办法可以设定拉开slider与横轴的间距的？

Reproduce the chart behavior in the linked CodeSandbox using G2Plot 2.3.18, comparing slider padding with and without outer chart padding. Trace the chart and slider layout from the reproduction; the issue is done when slider padding consistently creates visible space from the horizontal axis while outer padding is set.

Written by the indexing model from the issue text.