fix: use the third row of Matrix3X4 in Matrix3X4 * Matrix4X2

src/Maths/Silk.NET.Maths.Tests/Matrix3X4Tests.cs

@@ -0,0 +1,33 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+
+namespace Silk.NET.Maths.Tests
+{
+    public class Matrix3X4Tests
+    {
+        [Fact]
+        public void MultiplyByMatrix4X2UsesEveryRowOfTheLeftMatrix()
+        {
+            var left = new Matrix3X4<float>(
+                1, 2, 3, 4,
+                5, 6, 7, 8,
+                9, 10, 11, 12);
+            var right = new Matrix4X2<float>(
+                1, 2,
+                3, 4,
+                5, 6,
+                7, 8);
+
+            var result = left * right;
+
+            Assert.Equal(50f, result.M11);
+            Assert.Equal(60f, result.M12);
+            Assert.Equal(114f, result.M21);
+            Assert.Equal(140f, result.M22);
+            Assert.Equal(178f, result.M31);
+            Assert.Equal(220f, result.M32);
+        }
+    }
+}

src/Maths/Silk.NET.Maths/Matrix4X2.cs

@@ -342,7 +342,7 @@ public readonly bool IsIdentity
             return new(
                 value1.M11 * value2.Row1 + value1.M12 * value2.Row2 + value1.M13 * value2.Row3 + value1.M14 * value2.Row4,
                 value1.M21 * value2.Row1 + value1.M22 * value2.Row2 + value1.M23 * value2.Row3 + value1.M24 * value2.Row4,
-                value1.M21 * value2.Row1 + value1.M22 * value2.Row2 + value1.M23 * value2.Row3 + value1.M24 * value2.Row4
+                value1.M31 * value2.Row1 + value1.M32 * value2.Row2 + value1.M33 * value2.Row3 + value1.M34 * value2.Row4
             );
         }