为 UIVisualEffectView 添加阴影效果

此种方式不会在 UIVisualEffectView 下产生阴影颜色，而是围绕 UIVisualEffectView 生成的阴影效果。

/// UIVisualEffectView 设置阴影
    open func setup(_ blurEffectView: UIVisualEffectView, rect: CGRect, color: UIColor = .lightGray, offset: CGSize = .zero, opacity: Float = 0.5, radius: CGFloat = 3) {
        
        let shadowLayer = CALayer()
        
        let mutablePath = CGMutablePath()
        let maskLayer = CAShapeLayer()
        
        let rect = CGRect(x: 0, y: 0, width: UIScreen.main.bounds.width, height: blurEffectView.frame.height)
        let shadowPath = UIBezierPath(rect: rect).cgPath
        
        let shadowFrame = rect.insetBy(dx: -2 * radius, dy: -2 * radius).offsetBy(dx: offset.width, dy: offset.height)
        let shadowRect = CGRect(origin: .zero, size: shadowFrame.size)
        let shadowTransform = CGAffineTransform(translationX: -rect.origin.x - offset.width + 2 * radius, y: -rect.origin.y - offset.height + 2 * radius)
        
        shadowLayer.shadowPath = shadowPath
        shadowLayer.shadowOffset = offset
        shadowLayer.shadowOpacity = opacity
        shadowLayer.shadowRadius = radius
        shadowLayer.shadowColor = color.cgColor
        
        mutablePath.addRect(shadowRect)
        mutablePath.addPath(shadowLayer.shadowPath!, transform: shadowTransform)
        mutablePath.closeSubpath()
        
        maskLayer.frame = shadowFrame
        maskLayer.fillRule = kCAFillRuleEvenOdd
        maskLayer.path = mutablePath
        
        shadowLayer.mask = maskLayer
        
        blurEffectView.layer.superlayer?.insertSublayer(shadowLayer, above: blurEffectView.layer)
    }