Optimize the values for EMPTY rect. (#13470)

I am unsure if this needs changing, so let me know if I need to change
anything else.

# Objective

Fixes #13461.

## Solution

I applied the changes as suggested in the issue, and updated the doc
comments accordingly

## Testing

I don't think this needs too much testing, but there are no `cargo test`
failures.

crates/bevy_math/src/rects/irect.rs

@@ -20,9 +20,12 @@ pub struct IRect {
 
 impl IRect {
     /// An empty `IRect`, represented by maximum and minimum corner points
-    /// with all `i32::MAX` values.
+    /// with `max == IVec2::MIN` and `min == IVec2::MAX`, so the
+    /// rect has an extremely large negative size.
+    /// This is useful, because when taking a union B of a non-empty `IRect` A and
+    /// this empty `IRect`, B will simply equal A.
     pub const EMPTY: Self = Self {
-        max: IVec2::MAX,
+        max: IVec2::MIN,
         min: IVec2::MAX,
     };
     /// Create a new rectangle from two corner points.

crates/bevy_math/src/rects/rect.rs

@@ -20,10 +20,13 @@ pub struct Rect {
 
 impl Rect {
     /// An empty `Rect`, represented by maximum and minimum corner points
-    /// with all `f32::MAX` values.
+    /// at `Vec2::NEG_INFINITY` and `Vec2::INFINITY`, respectively.
+    /// This is so the `Rect` has a infinitely negative size.
+    /// This is useful, because when taking a union B of a non-empty `Rect` A and
+    /// this empty `Rect`, B will simply equal A.
     pub const EMPTY: Self = Self {
-        max: Vec2::MAX,
-        min: Vec2::MAX,
+        max: Vec2::NEG_INFINITY,
+        min: Vec2::INFINITY,
     };
     /// Create a new rectangle from two corner points.
     ///

crates/bevy_math/src/rects/urect.rs

@@ -20,9 +20,12 @@ pub struct URect {
 
 impl URect {
     /// An empty `URect`, represented by maximum and minimum corner points
-    /// with all `u32::MAX` values.
+    /// with `max == UVec2::MIN` and `min == UVec2::MAX`, so the
+    /// rect has an extremely large negative size.
+    /// This is useful, because when taking a union B of a non-empty `URect` A and
+    /// this empty `URect`, B will simply equal A.
     pub const EMPTY: Self = Self {
-        max: UVec2::MAX,
+        max: UVec2::MIN,
         min: UVec2::MAX,
     };
     /// Create a new rectangle from two corner points.