Merge pull request #1042 from jongleur1983/PorterDuffImprovement

small improvements on the PorterDuff functions

Author: JimBobSquarePants

src/ImageSharp/PixelFormats/PixelBlenders/PorterDuffFunctions.cs

@@ -164,7 +164,7 @@ public static Vector4 Over(Vector4 destination, Vector4 source, Vector4 blend)
             float srcW = source.W - blendW;
 
             // calculate final alpha
-            float alpha = dstW + srcW + blendW;
+            float alpha = dstW + source.W;
 
             // calculate final color
             Vector4 color = (destination * dstW) + (source * srcW) + (blend * blendW);
@@ -191,7 +191,7 @@ public static Vector4 Atop(Vector4 destination, Vector4 source, Vector4 blend)
             float dstW = destination.W - blendW;
 
             // calculate final alpha
-            float alpha = dstW + blendW;
+            float alpha = destination.W;
 
             // calculate final color
             Vector4 color = (destination * dstW) + (blend * blendW);