#542: add tests for multi-color gradients

somehow these don't look correct yet, containing hard edges sometimes.

tests/ImageSharp.Tests/Drawing/FillLinearGradientBrushTests.cs

@@ -158,5 +158,60 @@ namespace SixLabors.ImageSharp.Tests.Drawing
                 }
             }
         }
+        
+        [Theory]
+        [InlineData("a", 0, 0, 499, 499, new[] { 0f, .2f, .5f, .9f }, new[] { 0, 0, 1, 1 })]
+        [InlineData("b", 0, 499, 499, 0, new[] { 0f, 0.2f, 0.5f, 0.9f }, new[] { 0, 1, 2, 3 })]
+        [InlineData("c", 499, 499, 0, 0, new[] { 0f, 0.7f, 0.8f, 0.9f}, new[] { 0, 1, 2, 0 })]
+        [InlineData("d", 0, 0, 499, 499, new[] { 0f, .5f, 1f}, new[]{0, 1, 3})]
+        public void ArbitraryLinearGradientsProduceImages_VisualCheckOnly(
+            string filenameSuffix,
+            int startX, int startY,
+            int endX, int endY,
+            float[] stopPositions,
+            int[] stopColorCodes)
+        {
+            var colors = new Rgba32[]
+                             {
+                                 Rgba32.Navy,
+                                 Rgba32.LightGreen,
+                                 Rgba32.Yellow,
+                                 Rgba32.Red
+                             };
+            
+            var colorStops = new LinearGradientBrush<Rgba32>.ColorStop[stopPositions.Length];
+            for (int i = 0; i < stopPositions.Length; i++)
+            {
+                colorStops[i] = new LinearGradientBrush<Rgba32>.ColorStop(
+                    stopPositions[i],
+                    colors[stopColorCodes[i]]);
+            }
+            
+            
+            int size = 500;
+            int lastIndex = size - 1;
+            
+            
+            string path = TestEnvironment.CreateOutputDirectory("Fill", "LinearGradientBrush");
+            using (var image = new Image<Rgba32>(size, size))
+            {
+                LinearGradientBrush<Rgba32> unicolorLinearGradientBrush = 
+                    new LinearGradientBrush<Rgba32>(
+                        new SixLabors.Primitives.Point(startX, startY),
+                        new SixLabors.Primitives.Point(endX, endY),
+                        colorStops);
+                
+                image.Mutate(x => x.Fill(unicolorLinearGradientBrush));
+                image.Save($"{path}/arbitraryGradient_{filenameSuffix}.png");
+
+                using (PixelAccessor<Rgba32> sourcePixels = image.Lock())
+                {
+                    for (int i = 0; i < size; i++)
+                    {
+                        // it's diagonal, so for any (a, a) on the gradient line, for all (a-x, b+x) - +/- depending on the diagonal direction - must be the same color)
+                    }
+                }
+            }
+        }
     }
 }