Curtain is in front of Wheel

[dubiao]

https://github.com/AigeStudio/WheelPicker/blob/21cb7b1b887944423804705d68193e8a753ee75a/WheelPicker/src/main/java/com/aigestudio/wheelpicker/WheelPicker.java#L657-L663

一般选中效果是在滚轮当前值的后面，当成一个背景来用的。但这里好像是把选中效果放到轮子前面了。这会导致：

1. 当CurtainColor是一个不透明的颜色时，就看不到当前选中的值了。
2. 如果使用一个半透明的颜色，这又使得选中的view颜色有偏差。

可否考虑加一个参数，可以把 curtain放到轮子后面。（当然我明白这个变量名“窗帘”更像是庶在前面的，但从使用的角度来说，放到后面更像是常见的做法。） iOS 的滚轮如此。

当然，如果能增加更多参数使得可以更好地控制“窗帘”的边距圆角等样式，会更棒。（不求，但说）

The CurtainColor was drawed in front of select value causes the selected value was masked when the CurtainColor is a pure color without alpha. Or, if I use a color with alpha, it makes the selected value's color changing.

Can you please add a paramter allowing to move the curtain to behind the wheel. It will more looks like a "Selected Value" . (Of course, I know the "Curtain" means a cloth in front of some thing. But it is more usual usage to put it behind the wheel) See iOS picker wheel.