Added failing PixelRect snapping tests.

tests/Avalonia.Visuals.UnitTests/Media/PixelRectTests.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xunit;
+
+namespace Avalonia.Visuals.UnitTests.Media
+{
+    public class PixelRectTests
+    {
+        [Fact]
+        public void FromRect_Snaps_To_Device_Pixels()
+        {
+            var rect = new Rect(189, 189, 26, 164);
+            var result = PixelRect.FromRect(rect, 1.5);
+
+            Assert.Equal(new PixelRect(283, 283, 40, 247), result);
+        }
+
+        [Fact]
+        public void FromRect_Vector_Snaps_To_Device_Pixels()
+        {
+            var rect = new Rect(189, 189, 26, 164);
+            var result = PixelRect.FromRect(rect, new Vector(1.5, 1.5));
+
+            Assert.Equal(new PixelRect(283, 283, 40, 247), result);
+        }
+
+        [Fact]
+        public void FromRectWithDpi_Snaps_To_Device_Pixels()
+        {
+            var rect = new Rect(189, 189, 26, 164);
+            var result = PixelRect.FromRectWithDpi(rect, 144);
+
+            Assert.Equal(new PixelRect(283, 283, 40, 247), result);
+        }
+
+        [Fact]
+        public void FromRectWithDpi_Vector_Snaps_To_Device_Pixels()
+        {
+            var rect = new Rect(189, 189, 26, 164);
+            var result = PixelRect.FromRectWithDpi(rect, new Vector(144, 144));
+
+            Assert.Equal(new PixelRect(283, 283, 40, 247), result);
+        }
+    }
+}