Make Gizmos::rect_2d and primitive_2d draw the joint of the first corner of closed shapes

[uckelman]

Make Gizmos::rect_2d and primitive_2d draw the joint of the first corner of closed shapes

Objective

• Gizmos::rect_2d and primitive_2d do not draw the line joint between the first and last segments. This looks odd when the line joint is not GizmoLineJoint::None. (This affects Triangle2d, Rhombus, Rectangle, Polygon, and RegularPolygon.)
• Fixes Add explicit gizmo mechanism for drawing closed shapes #22081.

Solution

• Gizmos::rect_2d and primitive_2d draw using Gizmos::linestrip_2d, which doesn't draw a closed shape.
• In order to make linestrip_2d to draw a joint at a corner, that corner has to be in the points list with a predecessor and a successor.
• There is no predecessor for the first element in the list, and no successor when it is repeated as the last.
• Appending the second point to the list is enough to put the first point between two other points, which causes the joint to be rendered.
• I added a Gizmos::lineloop_2d function which handles the final joint for all closed shapes.
• Draw closed shapes with the lineloop_2d function, rather than manually adding closing vertices in each case.

Testing

• I tested my change against this demo:

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)
    );
}

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Showcase

Before:

After:

[alice-i-cecile]

Once this is merged, we should make a follow-up issue to remove the overdraw.