fixed broken Rectangle.Clip() extensions method (#998)

(rectangle.Right depends on rectangle.X, which has been changed before; unit test will fail with old implementation)

source/MonoGame.Extended/Rectangle.Extensions.cs

@@ -41,16 +41,18 @@ public static class RectangleExtensions
     /// <returns>The clipped rectangle, or <see cref="Rectangle.Empty"/> if the rectangles do not intersect.</returns>
     public static Rectangle Clip(this Rectangle rectangle, Rectangle clippingRectangle)
     {
-        var clip = clippingRectangle;
-        rectangle.X = clip.X > rectangle.X ? clip.X : rectangle.X;
-        rectangle.Y = clip.Y > rectangle.Y ? clip.Y : rectangle.Y;
-        rectangle.Width = rectangle.Right > clip.Right ? clip.Right - rectangle.X : rectangle.Width;
-        rectangle.Height = rectangle.Bottom > clip.Bottom ? clip.Bottom - rectangle.Y : rectangle.Height;
+        int left = Math.Max(rectangle.Left, clippingRectangle.Left);
+        int top = Math.Max(rectangle.Top, clippingRectangle.Top);
+        int right = Math.Min(rectangle.Right, clippingRectangle.Right);
+        int bottom = Math.Min(rectangle.Bottom, clippingRectangle.Bottom);
 
-        if (rectangle.Width <= 0 || rectangle.Height <= 0)
+        int width = right - left;
+        int height = bottom - top;
+
+        if (width <= 0 || height <= 0)
             return Rectangle.Empty;
 
-        return rectangle;
+        return new Rectangle(left, top, width, height);
     }
 
     /// <summary>

tests/MonoGame.Extended.Tests/RectangleExtensionsTests.cs

@@ -0,0 +1,20 @@
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Tests
+{
+    public class RectangleExtensionsTests
+    {
+        [Fact]
+        public void Clip_ReturnsIntersectionRectangle()
+        {
+            var rect = new Rectangle(0, 0, 10, 10);
+            var clip1 = new Rectangle(2, 2, 5, 5);
+            var clip2 = new Rectangle(2, 2, 15, 15);
+            var clip3 = new Rectangle(-2, -2, 5, 5);
+
+            Assert.Equal(new Rectangle(2, 2, 5, 5), rect.Clip(clip1));
+            Assert.Equal(new Rectangle(2, 2, 8, 8), rect.Clip(clip2));
+            Assert.Equal(new Rectangle(0, 0, 3, 3), rect.Clip(clip3));
+        }
+    }
+}