Update flecs::ref to store world instead of stage (#913)

* update ecs_get_ref_id() to handle staged world

Encountered a case using the C++ api where an assert() hit when using
`get_ref().get()`.  This was because the world inside the `flecs::ref`
was actually a stage, and not the world.

Updated `ecs_get_ref_id()` to convert stage to world during lookup.

* Revert "update ecs_get_ref_id() to handle staged world"

This reverts commit 1ca55e4.

* convert stage to world in flecs::ref constructor

From feedback on PR, moved the conversion of stage into workd to the
constructor for flecs::ref.

include/flecs/addons/cpp/ref.hpp

@@ -21,10 +21,13 @@ namespace flecs
  */
 template <typename T>
 struct ref {
-    ref(world_t *world, entity_t entity, flecs::id_t id = 0) 
-        : m_world( world )
-        , m_ref() 
+    ref(world_t *world, entity_t entity, flecs::id_t id = 0)
+        : m_ref()
     {
+        // the world we were called with may be a stage; convert it to a world
+        // here if that is the case
+        m_world = world ? const_cast<flecs::world_t *>(ecs_get_world(world))
+            : nullptr;
         if (!id) {
             id = _::cpp_type<T>::id(world);
         }

test/cpp_api/project.json

@@ -938,7 +938,8 @@
                 "non_const_ref",
                 "pair_ref",
                 "pair_ref_w_entity",
-                "pair_ref_second"
+                "pair_ref_second",
+                "from_stage"
             ]
         }, {
             "id": "Module",

test/cpp_api/src/Refs.cpp

@@ -115,3 +115,12 @@ void Refs_pair_ref_second() {
 
     test_int(e.get_second<Position>(tag)->x, 11);
 }
+
+void Refs_from_stage() {
+    flecs::world world;
+    flecs::world stage = world.get_stage(0); // get default stage
+    flecs::entity e = stage.entity().set<Position>({10, 20});
+    auto ref = e.get_ref<Position>();
+    test_int(ref->x, 10);
+    test_int(ref->y, 20);
+}