Fix drawing on transparent background

src/ImageSharp.Drawing/Processors/DrawPathProcessor.cs

@@ -110,7 +110,6 @@ namespace ImageSharp.Drawing.Processors
                                 Vector4 sourceVector = color.Color.ToVector4();
 
                                 Vector4 finalColor = Vector4BlendTransforms.PremultipliedLerp(backgroundVector, sourceVector, opacity);
-                                finalColor.W = backgroundVector.W;
 
                                 TColor packed = default(TColor);
                                 packed.PackFromVector4(finalColor);

src/ImageSharp.Drawing/Processors/FillRegionProcessor.cs

@@ -202,7 +202,6 @@ namespace ImageSharp.Drawing.Processors
                                 Vector4 sourceVector = applicator[x, y].ToVector4();
 
                                 Vector4 finalColor = Vector4BlendTransforms.PremultipliedLerp(backgroundVector, sourceVector, opacity);
-                                finalColor.W = backgroundVector.W;
 
                                 TColor packed = default(TColor);
                                 packed.PackFromVector4(finalColor);

tests/ImageSharp.Tests/Drawing/SolidPolygonTests.cs

@@ -32,7 +32,6 @@ namespace ImageSharp.Tests.Drawing
                 using (FileStream output = File.OpenWrite($"{path}/Simple.png"))
                 {
                     image
-                        .BackgroundColor(Color.Blue)
                         .FillPolygon(Color.HotPink, simplePath, new GraphicsOptions(true))
                         .Save(output);
                 }
@@ -45,7 +44,7 @@ namespace ImageSharp.Tests.Drawing
 
                     Assert.Equal(Color.HotPink, sourcePixels[50, 50]);
 
-                    Assert.Equal(Color.Blue, sourcePixels[2, 2]);
+                    Assert.NotEqual(Color.HotPink, sourcePixels[2, 2]);
                 }
             }
         }