`Stencil::Increment` does not seem to work.

[kaikalii]

Consider this simple test of increment stencil functionality:

use piston_window::*;

fn main() {
    let mut window: PistonWindow = WindowSettings::new("test", [800; 2]).build().unwrap();
    while let Some(event) = window.next() {
        window.draw_2d(&event, |Context { transform, .. }, g, _| {
            // Clear with black
            clear([0.0, 0.0, 0.0, 1.0], g);
            // Draw 2 stencil-incrementing circles
            Ellipse::new([1.0; 4]).draw(
                [0.0, 0.0, 400.0, 400.0],
                &DrawState::new_increment(),
                transform,
                g,
            );
            Ellipse::new([1.0; 4]).draw(
                [200.0, 0.0, 400.0, 400.0],
                &DrawState::new_increment(),
                transform,
                g,
            );
            // Draw a rectangle that should only fill the circles' overlapping region
            Rectangle::new([1.0; 4]).draw(
                [0.0, 0.0, 600.0, 400.0],
                &DrawState {
                    stencil: Some(draw_state::Stencil::Inside(2)),
                    ..Default::default()
                },
                transform,
                g,
            );
        });
    }
}

I compiled this with piston_window 0.113.0, which uses gfx_graphics 0.70.0, the latest version at time of writing. I think this code should draw a white shape that is the intersection of the two circles. However, it simply draws a black screen.

Is this a bug, or am I misusing the API?