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/CMakeLists.txt

@@ -466,6 +466,7 @@ set(minetest_SRCS
 	shader.cpp
 	sky.cpp
 	tile.cpp
+	wieldmesh.cpp
 	${minetest_SCRIPT_SRCS}
 )
 list(SORT minetest_SRCS)