Smoothstep 平滑比较

基于在该范围内的第三个值，平滑地在两个值之间进行混合，输出 0 到 1 之间的值。

可以把它想象成一个经过约束并平滑取值过的反转线性插值。

Blends smoothly between two values, based on where a third value is in that range, outputting values between 0 and 1. Think of it as a clamped inverse lerp with a smoothed output value.