Add a hasInstance to ResourceManager

This is needed to allow order-independent shutdown, due to the dumb
ownership behaviour for providers.

simgear/misc/ResourceManager.cxx

@@ -49,6 +49,11 @@ ResourceManager* ResourceManager::instance()
     return static_manager;
 }    
 
+bool ResourceManager::haveInstance()
+{
+    return static_manager != nullptr;
+}
+
 ResourceManager::~ResourceManager()
 {
     assert(this == static_manager);

simgear/misc/ResourceManager.hxx

@@ -47,6 +47,8 @@ public:
 
     static ResourceManager* instance();
 
+    static bool haveInstance();
+
     static void reset();
 
     /**