Implement WieldMeshSceneNode which improves wield mesh rendering

- Don't create and cache an extruded mesh for every (non-node) item.
  Instead use a single one per image resolution.

- For cubic nodes reuse a single wield mesh too

- Improve lighting of the wielded item

- Increase far value of wield mesh scene camera, fixes #1770

- Also includes some minor refactorings of Camera and GenericCAO.

src/clientobject.h

@@ -41,6 +41,7 @@ class ITextureSource;
 class IGameDef;
 class LocalPlayer;
 struct ItemStack;
+class WieldMeshSceneNode;
 
 class ClientActiveObject : public ActiveObject
 {
@@ -58,8 +59,10 @@ public:
 	virtual bool getCollisionBox(aabb3f *toset){return false;}
 	virtual bool collideWithObjects(){return false;}
 	virtual v3f getPosition(){return v3f(0,0,0);}
+	virtual scene::ISceneNode *getSceneNode(){return NULL;}
 	virtual scene::IMeshSceneNode *getMeshSceneNode(){return NULL;}
 	virtual scene::IAnimatedMeshSceneNode *getAnimatedMeshSceneNode(){return NULL;}
+	virtual WieldMeshSceneNode *getWieldMeshSceneNode(){return NULL;}
 	virtual scene::IBillboardSceneNode *getSpriteSceneNode(){return NULL;}
 	virtual bool isPlayer() const {return false;}
 	virtual bool isLocalPlayer() const {return false;}