Function render 

pub fn render(
    image: &mut Image,
    film: Option<&'static FilmStock>,
    selection: &Selection,
    options: &RenderOptions,
    output: &Path,
) -> Result<RenderOutcome, Error>

Develop a whole image and write it where asked.

The image is taken at the size it will be rendered at: the meter averages it, the grain is scaled by it, and the emulsions blur across it, so a caller that resizes afterwards would be rendering a different photograph. film overrides the stock the selection resolved to — that is how a .kalofilm file reaches the model without this crate knowing the format.