csharp: Fix Future callback lifetime

Assigning a method directly to a field expecting a delegate creates a
delegate on the fly. This delegate can be collected normally as any
collectable object. In dotnet the GC is more aggressive, causing this
delegate to be collected and C trying to call an invalid function.

To avoid this, we create a static delegate that will be passed to C. Its
lifetime will be tied to the static method it wraps.

src/bindings/mono/eina_mono/eina_promises.cs

@@ -295,12 +295,14 @@ public class Future
     private static IntPtr ThenRaw(IntPtr previous, ResolvedCb cb)
     {
         FutureDesc desc = new FutureDesc();
-        desc.cb = NativeResolvedCb;
+        desc.cb = NativeResolvedCbDelegate;
         GCHandle handle = GCHandle.Alloc(cb);
         desc.data = GCHandle.ToIntPtr(handle);
         return eina_future_then_from_desc(previous, desc);
     }
 
+    private static FutureCb NativeResolvedCbDelegate = new FutureCb(NativeResolvedCb);
+
     private static Eina.ValueNative NativeResolvedCb(IntPtr data, Eina.ValueNative value, IntPtr dead_future)
     {
         GCHandle handle = GCHandle.FromIntPtr(data);
@@ -340,7 +342,7 @@ public class Future
             for (; i < cbsList.Count(); i++)
             {
                 ResolvedCb cb = cbsList[i];
-                descs[i].cb = NativeResolvedCb;
+                descs[i].cb = NativeResolvedCbDelegate;
                 GCHandle handle = GCHandle.Alloc(cb);
                 descs[i].data = GCHandle.ToIntPtr(handle);
             }