Fix build error and a pair of warnings

The error was about casting between osg::Callback* and osg::NodeCallback*
The warnings are both about virtual classes with non-virtual destructors

apps/openmw/mwrender/animation.hpp

@@ -93,6 +93,7 @@ protected:
         boost::shared_ptr<float> mTimePtr;
 
     public:
+        virtual ~AnimationTime() { }
 
         void setTimePtr(boost::shared_ptr<float> time)
         { mTimePtr = time; }

apps/openmw/mwrender/sky.cpp

@@ -721,12 +721,12 @@ public:
                 if (stateset->getAttribute(osg::StateAttribute::MATERIAL))
                 {
                     SceneUtil::CompositeStateSetUpdater* composite = NULL;
-                    osg::NodeCallback* callback = node.getUpdateCallback();
+                    osg::NodeCallback* callback = dynamic_cast<osg::NodeCallback*>(node.getUpdateCallback());
                     while (callback)
                     {
                         if ((composite = dynamic_cast<SceneUtil::CompositeStateSetUpdater*>(callback)))
                             break;
-                        callback = callback->getNestedCallback();
+                        callback = dynamic_cast<osg::NodeCallback*>(callback->getNestedCallback());
                     }
 
                     if (composite)

apps/openmw/mwrender/weaponanimation.hpp

@@ -19,6 +19,8 @@ namespace MWRender
         float mStartTime;
     public:
         WeaponAnimationTime(Animation* animation) : mAnimation(animation), mStartTime(0) {}
+        virtual ~WeaponAnimationTime() { }
+
         void setGroup(const std::string& group);
         void updateStartTime();