Add GetBT709Luminance with vector test

tests/ImageSharp.Tests/Helpers/ImageMathsTests.cs

@@ -2,6 +2,11 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Numerics;
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.Memory;
+
 using Xunit;
 
 namespace SixLabors.ImageSharp.Tests.Helpers
@@ -131,6 +136,23 @@ namespace SixLabors.ImageSharp.Tests.Helpers
             Assert.Equal(expected, actual);
         }
 
+        [Theory]
+        [InlineData(0.2f, 0.7f, 0.1f, 256, 140)]
+        [InlineData(0.5f, 0.5f, 0.5f, 256, 128)]
+        [InlineData(0.5f, 0.5f, 0.5f, 65536, 32768)]
+        [InlineData(0.2f, 0.7f, 0.1f, 65536, 36069)]
+        public void GetBT709Luminance_WithVector4(float x, float y, float z, int luminanceLevels, int expected)
+        {
+            // arrange
+            var vector = new Vector4(x, y, z, 0.0f);
+
+            // act
+            int actual = ImageMaths.GetBT709Luminance(ref vector, luminanceLevels);
+
+            // assert
+            Assert.Equal(expected, actual);
+        }
+
         // TODO: We need to test all ImageMaths methods!
     }
-}
+}