Use the deferred virtual call mechanism to avoid the virtual

call bug in the constructor.

See docs/wrong_virtual_call.md for a description.

docs/wrong_virtual_call.md

@@ -0,0 +1,25 @@
+1) SizeNotifiable, defined as the following:
+
+    class SizeNotifiable<RegisterBehaviour, true> : private RegisterBehaviour, public SizeNotifiableBase { ... };
+
+gets constructed - it inherits from SizeNotifiableBase which declares NotifyResChanged() as virtual. It also inherits from RegisterBehaviour, which must provide a Register() method.
+
+2) In SizeNotifiable's constructor RegisterBehaviour::Register(this) gets called. Being in the constructor, virtual calls on this are UB.
+
+3) When RegisterBehaviour == AutoRegister, we have Register defined as the following:
+
+    void AutoRegister::Register (SizeNotifiableBase* parNotifiable) {
+        m_id = m_sdlmain->RegisterForResChange(parNotifiable);
+    }
+
+note that parNotifiable is received from SizeNotifiable's constructor, so it's a partially constructed object.
+
+4) RegisterForResChange() is defined as:
+
+    size_t SDLMain::RegisterForResChange (SizeNotifiableBase* parNotif) {
+        parNotif->NotifyResChanged(m_localData->sizeratio);
+        return m_localData->resChangeNotifList.Add(parNotif);
+    }
+
+parNotif is, again, the partly constructed object pointed by this in SizeNotifiable's constructor. NotifyResChanged() is a virtual method, thus the bug.
+Objects being registered need to be notified straight away since they have no clue about the current size held by SDLMain.