Make a function handle Float and Double

Using Swift
5

The FloatingPoint protocol *does* work with integer literals though, so once you take care of pow then you can write:

func srgb2linear<T: FloatingPoint> (_ s: T) -> T {
  if s <= (4045/100_000) {  // compare to 0.04045
    return s * (100/1292)   // divide by 12.92
  } else {
    return pow((s + 55/1000) * (1000/1055), 24/10)
//  return pow((s + 0.055) / 1.055, 2.4)
  }
}