Function render_image 

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

Develop and print an image in place, without writing anything.

This is render minus the file: the pixels stay in memory for a caller that has a window to put them in. A preview session renders the same pixels the print would, so a preview and the print it stands for cannot drift apart.