Add Meshable trait and implement meshing for 2D primitives (#11431)

# Objective

The first part of #10569, split up from #11007.

The goal is to implement meshing support for Bevy's new geometric
primitives, starting with 2D primitives. 3D meshing will be added in a
follow-up, and we can consider removing the old mesh shapes completely.

## Solution

Add a `Meshable` trait that primitives need to implement to support
meshing, as suggested by the
[RFC](https://github.com/bevyengine/rfcs/blob/main/rfcs/12-primitive-shapes.md#meshing).

```rust
/// A trait for shapes that can be turned into a [`Mesh`].
pub trait Meshable {
    /// The output of [`Self::mesh`]. This can either be a [`Mesh`]
    /// or a builder used for creating a [`Mesh`].
    type Output;

    /// Creates a [`Mesh`] for a shape.
    fn mesh(&self) -> Self::Output;
}
```

This PR implements it for the following primitives:

- `Circle`
- `Ellipse`
- `Rectangle`
- `RegularPolygon`
- `Triangle2d`

The `mesh` method typically returns a builder-like struct such as
`CircleMeshBuilder`. This is needed to support shape-specific
configuration for things like mesh resolution or UV configuration:

```rust
meshes.add(Circle { radius: 0.5 }.mesh().resolution(64));
```

Note that if no configuration is needed, you can even skip calling
`mesh` because `From<MyPrimitive>` is implemented for `Mesh`:

```rust
meshes.add(Circle { radius: 0.5 });
```

I also updated the `2d_shapes` example to use primitives, and tweaked
the colors to have better contrast against the dark background.

Before:

![Old 2D
shapes](https://github.com/bevyengine/bevy/assets/57632562/f1d8c2d5-55be-495f-8ed4-5890154b81ca)

After:

![New 2D
shapes](https://github.com/bevyengine/bevy/assets/57632562/f166c013-34b8-4752-800a-5517b284d978)

Here you can see the UVs and different facing directions: (taken from
#11007, so excuse the 3D primitives at the bottom left)

![UVs and facing
directions](https://github.com/bevyengine/bevy/assets/57632562/eaf0be4e-187d-4b6d-8fb8-c996ba295a8a)

---

## Changelog

- Added `bevy_render::mesh::primitives` module
- Added `Meshable` trait and implemented it for:
  - `Circle`
  - `Ellipse`
  - `Rectangle`
  - `RegularPolygon`
  - `Triangle2d`
- Implemented `Default` and `Copy` for several 2D primitives
- Updated `2d_shapes` example to use primitives
- Tweaked colors in `2d_shapes` example to have better contrast against
the (new-ish) dark background

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: Jondolf

Cargo.toml

@@ -402,7 +402,7 @@ doc-scrape-examples = true
 
 [package.metadata.example.2d_shapes]
 name = "2D Shapes"
-description = "Renders a rectangle, circle, and hexagon"
+description = "Renders simple 2D primitive shapes like circles and polygons"
 category = "2D Rendering"
 wasm = true
 

crates/bevy_math/src/primitives/dim2.rs

@@ -90,6 +90,13 @@ pub struct Circle {
 }
 impl Primitive2d for Circle {}
 
+impl Default for Circle {
+    /// Returns the default [`Circle`] with a radius of `0.5`.
+    fn default() -> Self {
+        Self { radius: 0.5 }
+    }
+}
+
 impl Circle {
     /// Create a new [`Circle`] from a `radius`
     #[inline(always)]
@@ -147,6 +154,15 @@ pub struct Ellipse {
 }
 impl Primitive2d for Ellipse {}
 
+impl Default for Ellipse {
+    /// Returns the default [`Ellipse`] with a half-width of `1.0` and a half-height of `0.5`.
+    fn default() -> Self {
+        Self {
+            half_size: Vec2::new(1.0, 0.5),
+        }
+    }
+}
+
 impl Ellipse {
     /// Create a new `Ellipse` from half of its width and height.
     ///
@@ -197,6 +213,15 @@ pub struct Plane2d {
 }
 impl Primitive2d for Plane2d {}
 
+impl Default for Plane2d {
+    /// Returns the default [`Plane2d`] with a normal pointing in the `+Y` direction.
+    fn default() -> Self {
+        Self {
+            normal: Direction2d::Y,
+        }
+    }
+}
+
 impl Plane2d {
     /// Create a new `Plane2d` from a normal
     ///
@@ -343,10 +368,19 @@ pub struct Triangle2d {
 }
 impl Primitive2d for Triangle2d {}
 
+impl Default for Triangle2d {
+    /// Returns the default [`Triangle2d`] with the vertices `[0.0, 0.5]`, `[-0.5, -0.5]`, and `[0.5, -0.5]`.
+    fn default() -> Self {
+        Self {
+            vertices: [Vec2::Y * 0.5, Vec2::new(-0.5, -0.5), Vec2::new(0.5, -0.5)],
+        }
+    }
+}
+
 impl Triangle2d {
     /// Create a new `Triangle2d` from points `a`, `b`, and `c`
     #[inline(always)]
-    pub fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
+    pub const fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
         Self {
             vertices: [a, b, c],
         }
@@ -438,6 +472,15 @@ pub struct Rectangle {
     pub half_size: Vec2,
 }
 
+impl Default for Rectangle {
+    /// Returns the default [`Rectangle`] with a half-width and half-height of `0.5`.
+    fn default() -> Self {
+        Self {
+            half_size: Vec2::splat(0.5),
+        }
+    }
+}
+
 impl Rectangle {
     /// Create a new `Rectangle` from a full width and height
     #[inline(always)]
@@ -559,9 +602,19 @@ pub struct RegularPolygon {
 }
 impl Primitive2d for RegularPolygon {}
 
+impl Default for RegularPolygon {
+    /// Returns the default [`RegularPolygon`] with six sides (a hexagon) and a circumradius of `0.5`.
+    fn default() -> Self {
+        Self {
+            circumcircle: Circle { radius: 0.5 },
+            sides: 6,
+        }
+    }
+}
+
 impl RegularPolygon {
     /// Create a new `RegularPolygon`
-    /// from the radius of the circumcircle and number of sides
+    /// from the radius of the circumcircle and a number of sides
     ///
     /// # Panics
     ///

crates/bevy_render/src/lib.rs

@@ -34,7 +34,7 @@ pub mod prelude {
             Projection,
         },
         color::Color,
-        mesh::{morph::MorphWeights, shape, Mesh},
+        mesh::{morph::MorphWeights, primitives::Meshable, shape, Mesh},
         render_resource::Shader,
         spatial_bundle::SpatialBundle,
         texture::{Image, ImagePlugin},

crates/bevy_render/src/mesh/mod.rs

@@ -1,10 +1,12 @@
 #[allow(clippy::module_inception)]
 mod mesh;
 pub mod morph;
+pub mod primitives;
 /// Generation for some primitive shape meshes.
 pub mod shape;
 
 pub use mesh::*;
+pub use primitives::*;
 
 use crate::{prelude::Image, render_asset::RenderAssetPlugin};
 use bevy_app::{App, Plugin};