获取折线（在 3D 空间中）上最接近当前鼠标位置的点。
折线就是由一组按顺序连接起来的点所定义的曲折线。ClosestPointToPolyLine 函数采用点数组定义的折线，并将折线"平展"到屏幕空间中。然后找到平展后的折线与当前鼠标位置（仍然在屏幕空间中）之间最近的点，并将其转换回原始 3D 折线上的 3D 点。这对于涉及使用鼠标操作任意 3D 形状的 Handle GUI 非常有用；按照 Unity 内置工具的惯用方法将 2D 鼠标运动转换到 3D 空间。可以使用此方法的一个示例是由一组路标定义的赛道，其中可以使用 ClosestPointToPolyLine 通过鼠标选择一个沿赛道线的位置。
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