Better benchmarks for ToVector4()

proving that the current API is fine

tests/ImageSharp.Benchmarks/General/PixelConversion/PixelConversion_ConvertToVector4.cs

@@ -75,9 +75,9 @@ namespace SixLabors.ImageSharp.Benchmarks.General.PixelConversion
         }
 
         // RESULTS:
-        //     Method | Count |     Mean |     Error |    StdDev | Scaled |
-        // ---------- |------ |---------:|----------:|----------:|-------:|
-        //  UseRetval |    32 | 94.99 ns | 1.1199 ns | 0.9352 ns |   1.00 |
-        //  UseCopyTo |    32 | 59.47 ns | 0.6104 ns | 0.5710 ns |   0.63 |
+        //     Method | Count |     Mean |    Error |   StdDev | Scaled |
+        // ---------- |------ |---------:|---------:|---------:|-------:|
+        //  UseRetval |    32 | 109.0 ns | 1.202 ns | 1.125 ns |   1.00 |
+        //  UseCopyTo |    32 | 108.6 ns | 1.151 ns | 1.020 ns |   1.00 |
     }
 }

tests/ImageSharp.Benchmarks/General/PixelConversion/PixelConversion_ConvertToVector4_AsPartOfCompositeOperation.cs

@@ -87,9 +87,9 @@ namespace SixLabors.ImageSharp.Benchmarks.General.PixelConversion
         }
 
         // RESULTS:
-        //     Method | Count |      Mean |     Error |    StdDev | Scaled |
-        // ---------- |------ |----------:|----------:|----------:|-------:|
-        //  UseRetval |    32 | 100.35 ns | 0.4844 ns | 0.4532 ns |   1.00 |
-        //  UseCopyTo |    32 |  53.95 ns | 0.1269 ns | 0.1125 ns |   0.54 |
+        //     Method | Count |     Mean |    Error |   StdDev | Scaled | ScaledSD |
+        // ---------- |------ |---------:|---------:|---------:|-------:|---------:|
+        //  UseRetval |    32 | 120.2 ns | 1.560 ns | 1.383 ns |   1.00 |     0.00 |
+        //  UseCopyTo |    32 | 121.7 ns | 2.439 ns | 2.281 ns |   1.01 |     0.02 |
     }
 }

tests/ImageSharp.Benchmarks/General/PixelConversion/TestRgba.cs

@@ -57,16 +57,15 @@ namespace SixLabors.ImageSharp.Benchmarks.General.PixelConversion
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public Vector4 ToVector4()
         {
-            return new Vector4(this.r, this.g, this.b, this.a);
+            return new Vector4(this.r, this.g, this.b, this.a) *  new Vector4(1f / 255f);
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void CopyToVector4(ref Vector4 dest)
         {
-            dest.X = this.r;
-            dest.Y = this.g;
-            dest.Z = this.b;
-            dest.W = this.a;
+            var tmp = new Vector4(this.r, this.g, this.b, this.a);
+            tmp *= new Vector4(1f / 255f);
+            dest = tmp;
         }
     }
 }