Revert color conversion and add test.

Author: maxvoxel8

src/ImageSharp.Drawing/Processing/PathGradientBrush.cs

@@ -281,9 +281,7 @@ public PathGradientBrushApplicator(
                         Vector4 pointColor = ((1 - u - v) * this.edges[0].StartColor) + (u * this.edges[0].EndColor) +
                                          (v * this.edges[2].StartColor);
 
-                        TPixel px = default;
-                        px.FromVector4(pointColor);
-                        return px;
+                        return new Color(pointColor).ToPixel<TPixel>();
                     }
 
                     var direction = Vector2.Normalize(point - this.center);
@@ -304,9 +302,7 @@ public PathGradientBrushApplicator(
 
                     var color = Vector4.Lerp(edgeColor, this.centerColor, ratio);
 
-                    TPixel pixel = default;
-                    pixel.FromVector4(color);
-                    return pixel;
+                    return new Color(color).ToPixel<TPixel>();
                 }
             }
 

tests/ImageSharp.Drawing.Tests/Drawing/FillPathGradientBrushTests.cs

@@ -53,6 +53,34 @@ public void FillTriangleWithDifferentColors<TPixel>(TestImageProvider<TPixel> pr
                 });
         }
 
+        [Theory]
+        [WithBlankImages(20, 20, PixelTypes.HalfSingle)]
+        public void FillTriangleWithGreyscale<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            provider.VerifyOperation(
+                TolerantComparer,
+                image =>
+                {
+                    PointF[] points = { new PointF(10, 0), new PointF(20, 20), new PointF(0, 20) };
+
+                    var c1 = new Rgba32();
+                    var c2 = new Rgba32();
+                    var c3 = new Rgba32();
+                    new HalfSingle(-1).ToRgba32(ref c1);
+                    new HalfSingle(0).ToRgba32(ref c2);
+                    new HalfSingle(1).ToRgba32(ref c3);
+
+                    Color[] colors = { new Color(c1), new Color(c2), new Color(c3)  };
+                    
+                    var brush = new PathGradientBrush(points, colors);
+
+                    image.Mutate(x => x.Fill(brush));
+                    image.DebugSave(provider, appendPixelTypeToFileName: false, appendSourceFileOrDescription: false);
+                });
+        }
+
+
         [Theory]
         [WithBlankImages(20, 20, PixelTypes.Rgba32)]
         public void FillTriangleWithDifferentColorsCenter<TPixel>(TestImageProvider<TPixel> provider)