Create missing constructors for 2D primitive mesh builders (#17291)

# Objective

- While all
[`MeshBuilder`](https://dev-docs.bevyengine.org/bevy/prelude/trait.MeshBuilder.html)s
can be created by first creating the primitive `bevy_math` type and
using
[`Meshable`](https://dev-docs.bevyengine.org/bevy/prelude/trait.Meshable.html),
most builders have their own `const` constructors that can be used
instead. Some builders are missing constructors, however, making them
unavailable in `const` contexts.

## Solution

- Add a `const` constructor for `RegularPolygonMeshBuilder`,
`RhombusMeshBuilder`, `Triangle2dMeshBuilder`, and
`RectangleMeshBuilder`.
- Add a note on the requirements of `ConvexPolygonMeshBuilder`, and
recommend using `ConvexPolygon::new().mesh()` instead.
- A constructor cannot easily be created for this type, since it
requires duplicating all of `ConvexPolygon::new()`'s verification code.
I may be able to work around this, but it requires touching a bit more
code surface. Opinions?

## Testing

Not much beyond CI! The changes are trivial enough that only a cursory
glance for typos and switched variables should be necessary.

## Note for Reviewers

Hi! I haven't directly used the types I modify in this PR beyond some
benchmarking work I did this morning. If you're familiar with these
types, please let me know if any of the constructors need additional
validation (or if the constructors shouldn't be there at all). Thanks!

---------

Co-authored-by: IQuick 143 <IQuick143cz@gmail.com>

crates/bevy_mesh/src/primitives/dim2.rs

@@ -399,6 +399,9 @@ impl From<CircularSegment> for Mesh {
 }
 
 /// A builder used for creating a [`Mesh`] with a [`ConvexPolygon`] shape.
+///
+/// You must verify that the `vertices` are not concave when constructing this type. You can
+/// guarantee this by creating a [`ConvexPolygon`] first, then calling [`ConvexPolygon::mesh()`].
 pub struct ConvexPolygonMeshBuilder<const N: usize> {
     pub vertices: [Vec2; N],
 }
@@ -452,6 +455,28 @@ pub struct RegularPolygonMeshBuilder {
     circumradius: f32,
     sides: u32,
 }
+
+impl RegularPolygonMeshBuilder {
+    /// Creates a new [`RegularPolygonMeshBuilder`] from the radius of a circumcircle and a number
+    /// of sides.
+    ///
+    /// # Panics
+    ///
+    /// Panics in debug mode if `circumradius` is negative, or if `sides` is less than 3.
+    pub const fn new(circumradius: f32, sides: u32) -> Self {
+        debug_assert!(
+            circumradius.is_sign_positive(),
+            "polygon has a negative radius"
+        );
+        debug_assert!(sides > 2, "polygon has less than 3 sides");
+
+        Self {
+            circumradius,
+            sides,
+        }
+    }
+}
+
 impl Meshable for RegularPolygon {
     type Output = RegularPolygonMeshBuilder;
 
@@ -726,6 +751,28 @@ pub struct RhombusMeshBuilder {
     half_diagonals: Vec2,
 }
 
+impl RhombusMeshBuilder {
+    /// Creates a new [`RhombusMeshBuilder`] from a horizontal and vertical diagonal size.
+    ///
+    /// # Panics
+    ///
+    /// Panics in debug mode if `horizontal_diagonal` or `vertical_diagonal` is negative.
+    pub const fn new(horizontal_diagonal: f32, vertical_diagonal: f32) -> Self {
+        debug_assert!(
+            horizontal_diagonal >= 0.0,
+            "rhombus has a negative horizontal size",
+        );
+        debug_assert!(
+            vertical_diagonal >= 0.0,
+            "rhombus has a negative vertical size"
+        );
+
+        Self {
+            half_diagonals: Vec2::new(horizontal_diagonal / 2.0, vertical_diagonal / 2.0),
+        }
+    }
+}
+
 impl MeshBuilder for RhombusMeshBuilder {
     fn build(&self) -> Mesh {
         let [hhd, vhd] = [self.half_diagonals.x, self.half_diagonals.y];
@@ -778,6 +825,16 @@ impl From<Rhombus> for Mesh {
 pub struct Triangle2dMeshBuilder {
     triangle: Triangle2d,
 }
+
+impl Triangle2dMeshBuilder {
+    /// Creates a new [`Triangle2dMeshBuilder`] from the points `a`, `b`, and `c`.
+    pub const fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
+        Self {
+            triangle: Triangle2d::new(a, b, c),
+        }
+    }
+}
+
 impl Meshable for Triangle2d {
     type Output = Triangle2dMeshBuilder;
 
@@ -785,6 +842,7 @@ impl Meshable for Triangle2d {
         Self::Output { triangle: *self }
     }
 }
+
 impl MeshBuilder for Triangle2dMeshBuilder {
     fn build(&self) -> Mesh {
         let vertices_3d = self.triangle.vertices.map(|v| v.extend(0.));
@@ -843,6 +901,22 @@ pub struct RectangleMeshBuilder {
     half_size: Vec2,
 }
 
+impl RectangleMeshBuilder {
+    /// Creates a new [`RectangleMeshBuilder`] from a full width and height.
+    ///
+    /// # Panics
+    ///
+    /// Panics in debug mode if `width` or `height` is negative.
+    pub const fn new(width: f32, height: f32) -> Self {
+        debug_assert!(width >= 0.0, "rectangle has a negative width");
+        debug_assert!(height >= 0.0, "rectangle has a negative height");
+
+        Self {
+            half_size: Vec2::new(width / 2.0, height / 2.0),
+        }
+    }
+}
+
 impl MeshBuilder for RectangleMeshBuilder {
     fn build(&self) -> Mesh {
         let [hw, hh] = [self.half_size.x, self.half_size.y];