Function process_image 

pub fn process_image<P: AsRef<Path>>(
    input_path: P,
    output_path: P,
    foreground_colors: Vec<Color>,
    background_color: Color,
    options: ProcessOptions,
) -> Result<()>

Process an image to remove its background.

Returns Err if options.strict_mode is set but foreground_colors is empty — strict mode requires at least one color to unmix against.