Fix cursor hotspot out of bounds when flipping (bevyengine#17571)

# Objective

- Fix off by one error introduced in
bevyengine#17540 causing:

```
Cursor image StrongHandle<Image>{ id: Index(AssetIndex { generation: 0, index: 3 }), path: Some(cursors/kenney_crosshairPack/Tilesheet/crosshairs_tilesheet_white.png) } is invalid: The specified hotspot (64, 64) is outside the image bounds (64x64).
```

- First PR commit and run shows the bug:
https://github.com/bevyengine/bevy/actions/runs/13009405866/job/36283507530?pr=17571
- Second PR commit fixes it.

## Solution

- Hotspot coordinates are 0-indexed, so we need to subtract 1 from the
width and height.

## Testing

- Fix the tests which included the off-by-one error in their expected
values.
- Consolidate the tests into a single test for brevity.
- Test round trip transform to ensure we can "undo" to get back to the
original value.
- Add a specific bounds test.
- Ran the example again and observed there are no more error logs:
`cargo run --example custom_cursor_image --features=custom_cursor`.

Author: mgi388

crates/bevy_winit/src/custom_cursor.rs

@@ -204,11 +204,16 @@ pub(crate) fn transform_hotspot(
 ) -> (u16, u16) {
     let hotspot_x = hotspot.0 as f32;
     let hotspot_y = hotspot.1 as f32;
+
     let (width, height) = (rect.width(), rect.height());
 
-    let hotspot_x = if flip_x { width - hotspot_x } else { hotspot_x };
+    let hotspot_x = if flip_x {
+        (width - 1.0).max(0.0) - hotspot_x
+    } else {
+        hotspot_x
+    };
     let hotspot_y = if flip_y {
-        height - hotspot_y
+        (height - 1.0).max(0.0) - hotspot_y
     } else {
         hotspot_y
     };
@@ -576,46 +581,26 @@ mod tests {
     );
 
     #[test]
-    fn test_transform_hotspot_no_flip() {
-        let hotspot = (10, 20);
-        let rect = Rect {
-            min: Vec2::ZERO,
-            max: Vec2::new(100.0, 200.0),
-        };
-        let transformed = transform_hotspot(hotspot, false, false, rect);
-        assert_eq!(transformed, (10, 20));
-    }
-
-    #[test]
-    fn test_transform_hotspot_flip_x() {
-        let hotspot = (10, 20);
-        let rect = Rect {
-            min: Vec2::ZERO,
-            max: Vec2::new(100.0, 200.0),
-        };
-        let transformed = transform_hotspot(hotspot, true, false, rect);
-        assert_eq!(transformed, (90, 20));
-    }
+    fn test_transform_hotspot() {
+        fn test(hotspot: (u16, u16), flip_x: bool, flip_y: bool, rect: Rect, expected: (u16, u16)) {
+            let transformed = transform_hotspot(hotspot, flip_x, flip_y, rect);
+            assert_eq!(transformed, expected);
+
+            // Round-trip test: Applying the same transformation again should
+            // reverse it.
+            let transformed = transform_hotspot(transformed, flip_x, flip_y, rect);
+            assert_eq!(transformed, hotspot);
+        }
 
-    #[test]
-    fn test_transform_hotspot_flip_y() {
-        let hotspot = (10, 20);
         let rect = Rect {
             min: Vec2::ZERO,
             max: Vec2::new(100.0, 200.0),
         };
-        let transformed = transform_hotspot(hotspot, false, true, rect);
-        assert_eq!(transformed, (10, 180));
-    }
 
-    #[test]
-    fn test_transform_hotspot_flip_both() {
-        let hotspot = (10, 20);
-        let rect = Rect {
-            min: Vec2::ZERO,
-            max: Vec2::new(100.0, 200.0),
-        };
-        let transformed = transform_hotspot(hotspot, true, true, rect);
-        assert_eq!(transformed, (90, 180));
+        test((10, 20), false, false, rect, (10, 20)); // no flip
+        test((10, 20), true, false, rect, (89, 20)); // flip X
+        test((10, 20), false, true, rect, (10, 179)); // flip Y
+        test((10, 20), true, true, rect, (89, 179)); // flip both
+        test((0, 0), true, true, rect, (99, 199)); // flip both (bounds check)
     }
 }