Method step_fall

src/main.rs:749–758

(&mut self, pos: Vec2, up: Vec2)

        self.snow_bg.step(&self.args, dir);
    }

    fn step(&mut self, dir: &Vec2) {
        let yrange: Box<dyn Iterator<Item = usize>> = match dir.y {
            -1 => Box::new((1..self.height).rev()), // down (process from bottom up)
            _ => Box::new(0..self.height - 1),      // up
        };
        let xrange: Box<dyn Iterator<Item = usize>> = match dir.x {
            -1 => Box::new((1..self.width).rev()),
            _ => Box::new(0..self.width - 1),
        };
        if dir.x == 0 {
            for y in yrange {
                for x in 0..self.width {