Fix the C++ build

Remove the Optional from the evaluation context passing for property
evaluation. Unfortunately there are nullptr uses left on the C++ side,
that need to be replaced with passing through.

api/sixtyfps-cpp/include/sixtyfps_properties.h

@@ -20,9 +20,9 @@ struct Property
 
     void set(const T &value) const { this->value = value; }
 
-    const T &get() const
+    const T &get(internal::EvaluationContext *context) const
     {
-        internal::sixtyfps_property_update(&inner, &value);
+        internal::sixtyfps_property_update(&inner, context, &value);
         return value;
     }