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/content_cao.h

@@ -70,6 +70,7 @@ private:
 	core::aabbox3d<f32> m_selection_box;
 	scene::IMeshSceneNode *m_meshnode;
 	scene::IAnimatedMeshSceneNode *m_animated_meshnode;
+	WieldMeshSceneNode *m_wield_meshnode;
 	scene::IBillboardSceneNode *m_spritenode;
 	scene::ITextSceneNode* m_textnode;
 	v3f m_position;
@@ -131,10 +132,14 @@ public:
 
 	v3f getPosition();
 
+	scene::ISceneNode *getSceneNode();
+
 	scene::IMeshSceneNode *getMeshSceneNode();
 
 	scene::IAnimatedMeshSceneNode *getAnimatedMeshSceneNode();
 
+	WieldMeshSceneNode *getWieldMeshSceneNode();
+
 	scene::IBillboardSceneNode *getSpriteSceneNode();
 
 	inline bool isPlayer() const