Add explicit gizmo mechanism for drawing closed shapes

[alice-i-cecile]

I don't love the double drawing either, but I didn't spot any underlying mechanism for drawing closed shapes.

Originally posted by @uckelman in #22080 (comment)

[uckelman]

I'm putting together a PR to add Gizmos::lineloop_2d. (As a start, to localize the problem to one place. The initial Gizmos::lineloop_2d will do the double draw. The step after that will be to stop doing the double draw.)

[uckelman]

Let's draw some shapes:

use bevy::prelude::*;

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .add_systems(Update, draw_shapes)
        .run();
}

fn setup(
    mut config_store: ResMut<GizmoConfigStore>,
    mut commands: Commands
)
{
    let (config, _) = config_store.config_mut::<DefaultGizmoConfigGroup>();
    config.line.width = 10.0;
    config.line.joints = GizmoLineJoint::Miter;

    commands.spawn(Camera2d);
}

fn draw_shapes(mut gizmos: Gizmos) {
    gizmos.rect_2d(
        Vec2::new(-200.0, 0.0),
        Vec2::new(50.0, 50.0),
        Color::srgb_u8(0xFF, 0, 0)
    );

    gizmos.primitive_2d(
        &Rhombus { half_diagonals: Vec2::new(20.0, 35.0) },
        Vec2::new(-100.0, 0.0),
        Color::srgb_u8(0xFF, 0, 0)
    );

    gizmos.primitive_2d(
        &Triangle2d { vertices: [Vec2::ZERO, Vec2::new(50.0, 0.), Vec2::new(0.0, 60.0) ] },
        Vec2::new(-20.0, -30.0),
        Color::srgb_u8(0xFF, 0, 0)
    );

    gizmos.primitive_2d(
        &Rectangle { half_size: Vec2::new(25.0, 25.0) },
        Vec2::new(100.0, 0.0),
        Color::srgb_u8(0xFF, 0, 0)
    );

     gizmos.primitive_2d(
        &RegularPolygon { sides: 5, circumcircle: Circle::new(40.0) },
        Vec2::new(200.0, 0.0),
        Color::srgb_u8(0xFF, 0, 0)
    );

    gizmos.primitive_2d(
        &Polygon { vertices: vec![Vec2::ZERO, Vec2::new(90.0, 0.), Vec2::new(75.0, 60.0), Vec2::new(15.0, 60.0) ] },
        Vec2::new(275.0, -30.0),
        Color::srgb_u8(0xFF, 0, 0)
    );
}

[uckelman]

I've opened PR #22085 to introduce Gizmos::lineloop_2d and use it for all the closed shapes.

lineloop_2d still draws the first segment twice, but has the advantage of doing in one spot instead of six.

The code in Gizmos appears to queue up the drawing. Where does the drawing actually happen? That's probably what we need to change to avoid the extra drawing.

Is it in DrawLineJointGizmo?