add full osx implementation for custom cursors.

src/Avalonia.Native/Cursor.cs

@@ -1,4 +1,5 @@
 using System;
+using System.IO;
 using Avalonia.Input;
 using Avalonia.Platform;
 using Avalonia.Native.Interop;
@@ -39,9 +40,22 @@ namespace Avalonia.Native
             return new AvaloniaNativeCursor( cursor );
         }
 
-        public ICursorImpl CreateCursor(IBitmapImpl cursor, PixelPoint hotSpot)
+        public unsafe ICursorImpl CreateCursor(IBitmapImpl cursor, PixelPoint hotSpot)
         {
-            throw new NotImplementedException();
+            using(var ms = new MemoryStream())
+            {
+                cursor.Save(ms);
+
+                var imageData = ms.ToArray();
+
+                fixed(void* ptr = imageData)
+                {
+                    var avnCursor = _native.CreateCustomCursor(ptr, new IntPtr(imageData.Length),
+                        new AvnPixelSize { Width = hotSpot.X, Height = hotSpot.Y });
+
+                    return new AvaloniaNativeCursor(avnCursor);
+                }
+            }
         }
     }
 }

src/Avalonia.Native/avn.idl

@@ -619,6 +619,7 @@ interface IAvnCursor : IUnknown
 interface IAvnCursorFactory : IUnknown
 {
      HRESULT GetCursor(AvnStandardCursorType cursorType, IAvnCursor** retOut);
+     HRESULT CreateCustomCursor (void* bitmapData, size_t length, AvnPixelSize hotPixel, IAvnCursor** retOut);
 }
 
 [uuid(60452465-8616-40af-bc00-042e69828ce7)]