Added failing DrawingContextImpl tests.

Skia `DrawingContextImpl` throws a NRE when passed a pen with 0 thickness.

tests/Avalonia.Skia.UnitTests/DrawingContextImplTests.cs

@@ -0,0 +1,30 @@
+using Avalonia.Media;
+using Avalonia.Skia.Helpers;
+using SkiaSharp;
+using Xunit;
+
+namespace Avalonia.Skia.UnitTests
+{
+    public class DrawingContextImplTests
+    {
+        [Fact]
+        public void DrawLine_With_Zero_Thickness_Pen_Does_Not_Throw()
+        {
+            var target = CreateTarget();
+            target.DrawLine(new Pen(Brushes.Black, 0), new Point(0, 0), new Point(10, 10));
+        }
+
+        [Fact]
+        public void DrawRectangle_With_Zero_Thickness_Pen_Does_Not_Throw()
+        {
+            var target = CreateTarget();
+            target.DrawRectangle(Brushes.Black, new Pen(Brushes.Black, 0), new RoundedRect(new Rect(0, 0, 100, 100), new CornerRadius(4)));
+        }
+
+        private DrawingContextImpl CreateTarget()
+        {
+            var canvas = new SKCanvas(new SKBitmap(100, 100));
+            return (DrawingContextImpl)DrawingContextHelper.WrapSkiaCanvas(canvas, new Vector(96, 96));
+        }
+    }
+}