Feature gate is_polygon_simple behind the alloc feature. (#16739)

CI was failing because `bevy_math` no longer compiled with `libcore`.
This was due to PR #15981. This commit fixes the issue by moving the
applicable functionality behind `#[cfg(feature = "alloc")]`.

Author: pcwalton

crates/bevy_math/src/primitives/dim2.rs

@@ -2,12 +2,15 @@ use core::f32::consts::{FRAC_1_SQRT_2, FRAC_PI_2, FRAC_PI_3, PI};
 use derive_more::derive::From;
 use thiserror::Error;
 
-use super::{polygon::is_polygon_simple, Measured2d, Primitive2d, WindingOrder};
+use super::{Measured2d, Primitive2d, WindingOrder};
 use crate::{
     ops::{self, FloatPow},
     Dir2, Vec2,
 };
 
+#[cfg(feature = "alloc")]
+use super::polygon::is_polygon_simple;
+
 #[cfg(feature = "bevy_reflect")]
 use bevy_reflect::{std_traits::ReflectDefault, Reflect};
 #[cfg(all(feature = "serialize", feature = "bevy_reflect"))]
@@ -1634,6 +1637,7 @@ impl<const N: usize> Polygon<N> {
     ///
     /// A polygon is simple if it is not self intersecting and not self tangent.
     /// As such, no two edges of the polygon may cross each other and each vertex must not lie on another edge.
+    #[cfg(feature = "alloc")]
     pub fn is_simple(&self) -> bool {
         is_polygon_simple(&self.vertices)
     }

crates/bevy_math/src/primitives/polygon.rs

@@ -1,5 +1,6 @@
-extern crate alloc;
-use alloc::collections::BTreeMap;
+#[cfg(feature = "alloc")]
+use alloc::{collections::BTreeMap, vec::Vec};
+
 use core::cmp::Ordering;
 
 use crate::Vec2;
@@ -58,10 +59,12 @@ fn xy_order(a: Vec2, b: Vec2) -> Ordering {
 }
 
 /// The event queue holds an ordered list of all events the [`SweepLine`] will encounter when checking the current polygon.
+#[cfg(feature = "alloc")]
 #[derive(Debug, Clone)]
 struct EventQueue {
     events: Vec<SweepLineEvent>,
 }
+#[cfg(feature = "alloc")]
 impl EventQueue {
     /// Initialize a new `EventQueue` with all events from the polygon represented by `vertices`.
     ///
@@ -143,11 +146,13 @@ struct SegmentOrder {
 ///
 /// It can be thought of as a vertical line sweeping from -X to +X across the polygon that keeps track of the order of the segments
 /// the sweep line is intersecting at any given moment.
+#[cfg(feature = "alloc")]
 #[derive(Debug, Clone)]
 struct SweepLine<'a> {
     vertices: &'a [Vec2],
     tree: BTreeMap<Segment, SegmentOrder>,
 }
+#[cfg(feature = "alloc")]
 impl<'a> SweepLine<'a> {
     fn new(vertices: &'a [Vec2]) -> Self {
         Self {
@@ -253,6 +258,7 @@ fn point_side(p1: Vec2, p2: Vec2, q: Vec2) -> f32 {
 ///
 /// The algorithm used is the Shamos-Hoey algorithm, a version of the Bentley-Ottman algorithm adapted to only detect whether any intersections exist.
 /// This function will run in O(n * log n)
+#[cfg(feature = "alloc")]
 pub fn is_polygon_simple(vertices: &[Vec2]) -> bool {
     if vertices.len() < 3 {
         return true;