[Feature] optimize the position of poptip

[xile611]

What problem does this feature solve?

1. poptip position: 'auto'

2. poptip position: 'bottom'

What does the proposed API look like?

1. poptip 组件在计算位置的时候，能够尽量不遮挡原始的图形
2. poptip 组件可以考虑接收一个 viewBox，将poptip组件尽量限制在viewBox内部进行展示