提供获取当前可视范围的中心点的坐标的api

[w582745483]

功能描述

不管画布怎么平移和缩放，动态加载节点时，希望位置能在可视范围内的中心

期望解决方案

希望能提供获取可视范围的中心点的坐标的接口能力

[x6-bot]

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