FastCPUDrawRect.swift

    override func draw(_ rect: CGRect) {
        super.draw(rect)
        guard let context = UIGraphicsGetCurrentContext() else { return }
        
        // draw the flattened image if it exists
        if let image = flattenedImage {
            image.draw(in: self.bounds)
        }
        
        context.setStrokeColor(lineColor.cgColor)
        context.setLineWidth(lineWidth)
        context.setLineCap(.round)
        
        for (index, point) in line.enumerated() {
            if index == 0 {
                context.move(to: point)
            } else {
                context.addLine(to: point)
            }
        }
        context.strokePath()
    }