MultiView: tooltip显示不完整

* **G2Plot Version**:
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当数据点靠近canvas上边界的时候，hover上去tooltip的定位没有自动加上offset导致tooltip渲染出界。在其他的图表中都有做自动加上offset这样的优化，但是multiview还没有优化处理。
截图如下：

Start by reproducing the MultiView tooltip with a data point near the canvas top, then compare its positioning with other chart types that already apply an offset. Trace the MultiView tooltip entry point and verify the fix by confirming that the tooltip remains fully within the canvas boundary.

Written by the indexing model from the issue text.