Modifies saturation and optionally pixelates `src`, placing the result in `dest`.
The `src` and `dest` pixbufs must have the same image format, size, and rowstride.
The `src` and `dest` arguments may be the same pixbuf with no ill effects.
If `saturation` is 1.0 then saturation is not changed. If it's less than 1.0, saturation is reduced (the image turns toward grayscale); if greater than 1.0, saturation is increased (the image gets more vivid colors).
If `pixelate` is `TRUE`, then pixels are faded in a checkerboard pattern to create a pixelated image.
place to write modified version of this
whether to pixelate