Add test coving subpixel counter

tests/ImageSharp.Tests/Drawing/FillRegionProcessorTests.cs

@@ -0,0 +1,46 @@
+
+namespace ImageSharp.Tests.Drawing
+{
+    using System;
+    using System.IO;
+    using ImageSharp;
+    using ImageSharp.Drawing.Brushes;
+    using Processing;
+    using System.Collections.Generic;
+    using Xunit;
+    using ImageSharp.Drawing;
+    using System.Numerics;
+    using SixLabors.Shapes;
+    using ImageSharp.Drawing.Processors;
+    using ImageSharp.Drawing.Pens;
+    using Moq;
+    using System.Collections.Immutable;
+
+    public class FillRegionProcessorTests
+    {
+        [Theory]
+        [InlineData(true, 1, 4)]
+        [InlineData(true, 2, 4)]
+        [InlineData(true, 5, 5)]
+        [InlineData(true, 8, 8)]
+        [InlineData(false, 8, 4)] 
+        [InlineData(false, 16, 4)] // we always do 4 sub=pixels when antialising is off.
+        public void MinimumAntialiasSubpixelDepth(bool antialias, int antialiasSubpixelDepth, int expectedAntialiasSubpixelDepth)
+        {
+            var bounds = new ImageSharp.Rectangle(0, 0, 1, 1);
+
+            Mock<IBrush<Color>> brush = new Mock<IBrush<Color>>();
+            Mock<Region> region = new Mock<Region>();
+            region.Setup(x => x.Bounds).Returns(bounds);
+
+            GraphicsOptions options = new GraphicsOptions(antialias) {
+                AntialiasSubpixelDepth = 1
+            };
+            FillRegionProcessor<Color> processor = new FillRegionProcessor<Color>(brush.Object, region.Object, options);
+            Image img = new Image(1, 1);
+            processor.Apply(img, bounds);
+
+            region.Verify(x => x.Scan(It.IsAny<float>(), It.IsAny<float[]>(), It.IsAny<int>(), It.IsAny<int>()), Times.Exactly(4));
+        }
+    }
+}