Update YogaConfig

Summary:
- Bugfix: Retain managed YogaConfig to prevent releasing unmanaged YogaConfig
- Use the same YogaConfig in the copy constructor
- Expose Set/GetUseWebDefaults APIs
- Split YogaConfig out from YogaNode.cs
Closes https://github.com/facebook/yoga/pull/496

Reviewed By: emilsjolander

Differential Revision: D4796178

Pulled By: splhack

fbshipit-source-id: cafabdc051ca914af547acbbf3d2246a5618e8bb

csharp/Facebook.Yoga/Native.cs

@@ -149,6 +149,19 @@ namespace Facebook.Yoga
             YGConfigHandle config,
             YogaExperimentalFeature feature);
 
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGConfigSetUseWebDefaults(
+            YGConfigHandle config,
+            bool useWebDefaults);
+
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern bool YGConfigGetUseWebDefaults(YGConfigHandle config);
+
+        [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
+        public static extern void YGConfigSetPointScaleFactor(
+            YGConfigHandle config,
+            float pixelsInPoint);
+
         [DllImport(DllName, ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
         public static extern void YGNodeInsertChild(
             YGNodeHandle node,