Add parser for spline easing

src/Avalonia.Animation/Easing/Easing.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Globalization;
 using System.Linq;
 
 namespace Avalonia.Animation.Easings
@@ -25,6 +26,37 @@ namespace Avalonia.Animation.Easings
         /// <returns>Returns the instance of the parsed type.</returns>
         public static Easing Parse(string e)
         {
+            if (e.Contains(','))
+            {
+                var k = e.Split(',');
+
+                if (k.Count() != 4)
+                    throw new FormatException($"SplineEasing only accepts exactly 4 arguments.");
+                var splineEase = new SplineEasing();
+
+                if (double.TryParse(k[0], NumberStyles.Any, CultureInfo.InvariantCulture, out var x1))
+                    splineEase.X1 = x1;
+                else
+                    throw new FormatException($"Invalid SplineEasing control point X1 value: {k[0]}");
+
+                if (double.TryParse(k[1], NumberStyles.Any, CultureInfo.InvariantCulture, out var y1))
+                    splineEase.Y1 = y1;
+                else
+                    throw new FormatException($"Invalid SplineEasing control point Y1 value: {k[1]}");
+
+                if (double.TryParse(k[2], NumberStyles.Any, CultureInfo.InvariantCulture, out var x2))
+                    splineEase.X2 = x2;
+                else
+                    throw new FormatException($"Invalid SplineEasing control point Y1 value: {k[2]}");
+
+                if (double.TryParse(k[3], NumberStyles.Any, CultureInfo.InvariantCulture, out var y2))
+                    splineEase.Y2 = y2;
+                else
+                    throw new FormatException($"Invalid SplineEasing control point Y1 value: {k[3]}");
+
+                return splineEase;
+            }
+
             if (_easingTypes == null)
             {
                 _easingTypes = new Dictionary<string, Type>();