Fix incorrect gradient color-stop calculation (SixLabors#864)

* Fix incorrect gradient color-stop calculation

* Update submodule

* Add multi-stop gradient test

* Add missing reference image

Author: Poyo

src/ImageSharp.Drawing/Processing/GradientBrushBase{TPixel}.cs

@@ -121,7 +121,7 @@ protected GradientBrushApplicatorBase(
                     {
                         var fromAsVector = from.Color.ToVector4();
                         var toAsVector = to.Color.ToVector4();
-                        float onLocalGradient = (positionOnCompleteGradient - from.Ratio) / to.Ratio;
+                        float onLocalGradient = (positionOnCompleteGradient - from.Ratio) / (to.Ratio - from.Ratio);
 
                         // TODO: this should be changeble for different gradienting functions
                         Vector4 result = PorterDuffFunctions.NormalSrcOver(

tests/ImageSharp.Tests/Drawing/FillLinearGradientBrushTests.cs

@@ -350,5 +350,52 @@ public void ArbitraryGradients<TPixel>(
                 false,
                 false);
         }
+
+        [Theory]
+        [WithBlankImages(200, 200, PixelTypes.Rgba32, 0, 0, 199, 199, new[] { 0f, .25f, .5f, .75f, 1f }, new[] { 0, 1, 2, 3, 4 })]
+        public void MultiplePointGradients<TPixel>(
+            TestImageProvider<TPixel> provider,
+            int startX, int startY,
+            int endX, int endY,
+            float[] stopPositions,
+            int[] stopColorCodes)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            TPixel[] colors =
+            {
+                NamedColors<TPixel>.Black, NamedColors<TPixel>.Blue, NamedColors<TPixel>.Red,
+                NamedColors<TPixel>.White, NamedColors<TPixel>.Lime
+            };
+
+            var coloringVariant = new StringBuilder();
+            var colorStops = new ColorStop<TPixel>[stopPositions.Length];
+
+            for (int i = 0; i < stopPositions.Length; i++)
+            {
+                TPixel color = colors[stopColorCodes[i % colors.Length]];
+                float position = stopPositions[i];
+                colorStops[i] = new ColorStop<TPixel>(position, color);
+                Rgba32 rgba = default;
+                color.ToRgba32(ref rgba);
+                coloringVariant.AppendFormat(CultureInfo.InvariantCulture, "{0}@{1};", rgba.ToHex(), position);
+            }
+
+            FormattableString variant = $"({startX},{startY})_TO_({endX},{endY})__[{coloringVariant}]";
+
+            provider.VerifyOperation(
+                image =>
+                {
+                    var unicolorLinearGradientBrush = new LinearGradientBrush<TPixel>(
+                        new SixLabors.Primitives.Point(startX, startY),
+                        new SixLabors.Primitives.Point(endX, endY),
+                        GradientRepetitionMode.None,
+                        colorStops);
+
+                    image.Mutate(x => x.Fill(unicolorLinearGradientBrush));
+                },
+                variant,
+                false,
+                false);
+        }
     }
 }