[spritekit] Fix SKActionTimingFunction delegate signature. Fixes #3944 (#4024)

https://github.com/xamarin/xamarin-macios/issues/3944

src/SpriteKit/ObsoleteCompat.cs

@@ -21,7 +21,7 @@ namespace SpriteKit {
 			return FalloffBy (to, duration);
 		}
 
-		[Obsolete ("Use the 'TimingFunction' property.")]
+		[Obsolete ("Use the 'TimingFunction2' property.")]
 		public virtual void SetTimingFunction (SKActionTimingFunction timingFunction)
 		{
 			TimingFunction = timingFunction;

src/SpriteKit/SKAction.cs

@@ -12,6 +12,12 @@ using CoreGraphics;
 using CoreFoundation;
 
 namespace SpriteKit {
+
+#if !XAMCORE_4_0
+	[Obsolete ("Use 'SKActionTimingFunction2' instead.")]
+	public delegate void SKActionTimingFunction (float /* float, not CGFloat */ time);
+#endif
+
 #if XAMCORE_2_0 || !MONOMAC
 	public partial class SKAction {
 
@@ -32,6 +38,10 @@ namespace SpriteKit {
 		{
 			return Run (block, queue);
 		}
+#endif
+#if !XAMCORE_4_0
+		[Obsolete ("Use 'TimingFunction2' instead.")]
+		public virtual SKActionTimingFunction TimingFunction { get; set; }
 #endif
 	}
 #endif

src/spritekit.cs

@@ -68,7 +68,7 @@ namespace SpriteKit {
 #endif
 
 	delegate void SKNodeChildEnumeratorHandler (SKNode node, out bool stop);
-	delegate void SKActionTimingFunction (float /* float, not CGFloat */ time);
+	delegate float SKActionTimingFunction2 (float /* float, not CGFloat */ time);
 
 	[Watch (3,0)]
 	[iOS (8,0), Mac (10,10, onlyOn64 : true)]
@@ -2324,7 +2324,7 @@ namespace SpriteKit {
 
 		[iOS (8,0), Mac (10,10)]
 		[NullAllowed, Export ("timingFunction", ArgumentSemantic.Assign)]
-		SKActionTimingFunction TimingFunction { get; set; }
+		SKActionTimingFunction2 TimingFunction2 { get; set; }
 
 		[iOS (8,0), Mac(10,10)]
 		[Static, Export ("falloffBy:duration:")]