Workaround for P/Invoke AccessViolationException

Summary:
The issue is on ARM builds for Windows UWP. For the C# P/Invoke API wrapper, any native method that returns the YogaValue struct throws the AccessViolationException. The issue is not with structs in general, as returning the YogaSize / YGSize struct works fine. The issue seems to be limited to structs that have an enum member.

I tried a number of things to resolve the issue without changing the underlying native API for Windows. I read the ARM documentation and saw reference to variable enum sizes based on the number of enum members, so I tried to use a number of different UnmanagedType values in a [MarsalAs()] attribute on the enum member of the C# struct declaration.

What ultimately worked was to return a pointer to the location of the struct, and use the System.Runtime.InteropServices.PtrToStructure API to read the struct data from that pointer. I added a few new macros that will return the struct address on Windows only, other builds are not affected.

Note, I have not tested the impact of this ch
Closes https://github.com/facebook/yoga/pull/459

Reviewed By: emilsjolander

Differential Revision: D4652278

Pulled By: splhack

fbshipit-source-id: bf7ada4da1781e3f813b3ba331974b7bded476d9

csharp/Facebook.Yoga/YogaValue.cs

@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
+using System;
 using System.Runtime.InteropServices;
 
 namespace Facebook.Yoga
@@ -92,5 +93,17 @@ namespace Facebook.Yoga
         {
             return Point(pointValue);
         }
+
+#if WINDOWS_UWP_ARM
+        internal static YogaValue MarshalValue(IntPtr ptr)
+        {
+            return Marshal.PtrToStructure<YogaValue>(ptr);
+        }
+#else
+        internal static YogaValue MarshalValue(YogaValue value)
+        {
+            return value;
+        }
+#endif
     }
 }