the function in MapFun should be a function that operates an image

[johnnychen94]

Currently, we need to pass a function that operates on single pixel to MapFun, many operations become invalid because of this; many available image processing algorithms accept only image (not pixel).

Pixel operations can be just broadcasted to become valid, e.g.,

- augment(img, MapFun(px -> px - RGBA(0.5, 0.3, 0.7, 0.0)))
+ augment(img, MapFun(px -> px .- RGBA(0.5, 0.3, 0.7, 0.0)))

A usage of this could be:

pl = ConvertEltype(Gray{Float32}) |>
     MapFun(AdditiveWhiteGaussianNoise(0.1)) |> # ImageNoise.jl
     Either(NoOp(), Rotate90(), Rotate180(), Rotate270()) |>
     Either(0.5=>NoOp(), 0.25=>FlipX(), 0.25=>FlipY())