Add winding order for Triangle2d (bevyengine#10620)

# Objective

This PR adds some helpers for `Triangle2d` to work with its winding
order. This could also be extended to polygons (and `Triangle3d` once
it's added).

## Solution

- Add `WindingOrder` enum with `Clockwise`, `Counterclockwise` and
`Invalid` variants
- `Invalid` is for cases where the winding order can not be reliably
computed, i.e. the points lie on a single line and the area is zero
- Add `Triangle2d::winding_order` method that uses a signed surface area
to determine the winding order
- Add `Triangle2d::reverse` method that reverses the winding order by
swapping the second and third vertices

The API looks like this:

```rust
let mut triangle = Triangle2d::new(
    Vec2::new(0.0, 2.0),
    Vec2::new(-0.5, -1.2),
    Vec2::new(-1.0, -1.0),
);
assert_eq!(triangle.winding_order(), WindingOrder::Clockwise);

// Reverse winding order
triangle.reverse();
assert_eq!(triangle.winding_order(), WindingOrder::Counterclockwise);
```

I also added tests to make sure the methods work correctly. For now,
they live in the same file as the primitives.

## Open questions

- Should it be `Counterclockwise` or `CounterClockwise`? The first one
is more correct but perhaps a bit less readable. Counter-clockwise is
also a valid spelling, but it seems to be a lot less common than
counterclockwise.
- Is `WindingOrder::Invalid` a good name? Parry uses
`TriangleOrientation::Degenerate`, but I'm not a huge fan, at least as a
non-native English speaker. Any better suggestions?
- Is `WindingOrder` fine in `bevy_math::primitives`? It's not specific
to a dimension, so I put it there for now.

crates/bevy_math/src/primitives/dim2.rs

@@ -1,4 +1,4 @@
-use super::Primitive2d;
+use super::{Primitive2d, WindingOrder};
 use crate::Vec2;
 
 /// A normalized vector pointing in a direction in 2D space
@@ -174,20 +174,40 @@ impl BoxedPolyline2d {
 }
 
 /// A triangle in 2D space
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq)]
 pub struct Triangle2d {
     /// The vertices of the triangle
     pub vertices: [Vec2; 3],
 }
 impl Primitive2d for Triangle2d {}
 
 impl Triangle2d {
-    /// Create a new `Triangle2d` from `a`, `b`, and `c`,
+    /// Create a new `Triangle2d` from points `a`, `b`, and `c`
     pub fn new(a: Vec2, b: Vec2, c: Vec2) -> Self {
         Self {
             vertices: [a, b, c],
         }
     }
+
+    /// Get the [`WindingOrder`] of the triangle
+    #[doc(alias = "orientation")]
+    pub fn winding_order(&self) -> WindingOrder {
+        let [a, b, c] = self.vertices;
+        let area = (b - a).perp_dot(c - a);
+        if area > f32::EPSILON {
+            WindingOrder::CounterClockwise
+        } else if area < -f32::EPSILON {
+            WindingOrder::Clockwise
+        } else {
+            WindingOrder::Invalid
+        }
+    }
+
+    /// Reverse the [`WindingOrder`] of the triangle
+    /// by swapping the second and third vertices
+    pub fn reverse(&mut self) {
+        self.vertices.swap(1, 2);
+    }
 }
 
 /// A rectangle primitive
@@ -298,3 +318,37 @@ impl RegularPolygon {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn triangle_winding_order() {
+        let mut cw_triangle = Triangle2d::new(
+            Vec2::new(0.0, 2.0),
+            Vec2::new(-0.5, -1.2),
+            Vec2::new(-1.0, -1.0),
+        );
+        assert_eq!(cw_triangle.winding_order(), WindingOrder::Clockwise);
+
+        let ccw_triangle = Triangle2d::new(
+            Vec2::new(0.0, 2.0),
+            Vec2::new(-1.0, -1.0),
+            Vec2::new(-0.5, -1.2),
+        );
+        assert_eq!(ccw_triangle.winding_order(), WindingOrder::CounterClockwise);
+
+        // The clockwise triangle should be the same as the counterclockwise
+        // triangle when reversed
+        cw_triangle.reverse();
+        assert_eq!(cw_triangle, ccw_triangle);
+
+        let invalid_triangle = Triangle2d::new(
+            Vec2::new(0.0, 2.0),
+            Vec2::new(0.0, -1.0),
+            Vec2::new(0.0, -1.2),
+        );
+        assert_eq!(invalid_triangle.winding_order(), WindingOrder::Invalid);
+    }
+}

crates/bevy_math/src/primitives/mod.rs

@@ -12,3 +12,16 @@ pub trait Primitive2d {}
 
 /// A marker trait for 3D primitives
 pub trait Primitive3d {}
+
+/// The winding order for a set of points
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum WindingOrder {
+    /// A clockwise winding order
+    Clockwise,
+    /// A counterclockwise winding order
+    CounterClockwise,
+    /// An invalid winding order indicating that it could not be computed reliably.
+    /// This often happens in *degenerate cases* where the points lie on the same line
+    #[doc(alias = "Degenerate")]
+    Invalid,
+}