CActor: Make Render() a non-const function

A few implementations of Render() contain const-casts nested within its
their call hierarchy to get around the fact that this function is marked
const. We can just make the member function non-const to allow removal
of these casts in follow up changes.

Runtime/Weapon/CWeapon.cpp

@@ -37,7 +37,7 @@ void CWeapon::Think(float dt, CStateManager& mgr) {
   CEntity::Think(dt, mgr);
 }
 
-void CWeapon::Render(const CStateManager&) const {
+void CWeapon::Render(CStateManager&) {
   // Empty
 }