add stubs for headless/mock

src/Avalonia.Headless/HeadlessPlatformRenderInterface.cs

@@ -104,6 +104,9 @@ namespace Avalonia.Headless
             }
 
             public Rect Bounds { get; set; }
+            
+            public double ContourLength { get; } = 0;
+            
             public virtual bool FillContains(Point point) => Bounds.Contains(point);
 
             public Rect GetRenderBounds(IPen pen)
@@ -126,6 +129,19 @@ namespace Avalonia.Headless
 
             public ITransformedGeometryImpl WithTransform(Matrix transform) =>
                 new HeadlessTransformedGeometryStub(this, transform);
+
+            public bool TryGetPointAtDistance(double distance, out Point point)
+            {
+                point = new Point();
+                return false;
+            }
+
+            public bool TryGetPointAndTangentAtDistance(double distance, out Point point, out Point tangent)
+            {
+                point = new Point();
+                tangent = new Point();
+                return false;
+            }
         }
 
         class HeadlessTransformedGeometryStub : HeadlessGeometryStub, ITransformedGeometryImpl

tests/Avalonia.UnitTests/MockStreamGeometryImpl.cs

@@ -30,6 +30,8 @@ namespace Avalonia.UnitTests
         public IGeometryImpl SourceGeometry { get; }
 
         public Rect Bounds => _context.CalculateBounds();
+        
+        public double ContourLength { get; }
 
         public Matrix Transform { get; }
 
@@ -69,6 +71,19 @@ namespace Avalonia.UnitTests
             return new MockStreamGeometryImpl(transform, _context);
         }
 
+        public bool TryGetPointAtDistance(double distance, out Point point)
+        {
+            point = new Point();
+            return false;
+        }
+
+        public bool TryGetPointAndTangentAtDistance(double distance, out Point point, out Point tangent)
+        {
+            point = new Point();
+            tangent = new Point();
+            return false;
+        }
+
         class MockStreamGeometryContext : IStreamGeometryContextImpl
         {
             private List<Point> points = new List<Point>();