如何使用Core Graphics渲染到屏幕外位图然后blit到屏幕

let colorSpace = CGColorSpaceCreateDeviceRGB()
let bitmapInfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.premultipliedLast.rawValue)
let context = CGContext(data: nil, width: Int(width), height: Int(height), bitsPerComponent: 8, bytesPerRow: 0, space: colorSpace, bitmapInfo: bitmapInfo.rawValue)

// 绘制内容
context?.setFillColor(UIColor.red.cgColor)
context?.fill(CGRect(x: 0, y: 0, width: width, height: height))

let image = context?.makeImage()

let rect = CGRect(x: 0, y: 0, width: width, height: height)
image?.draw(in: rect)