Improve getPointedThing() (#4346)

* Improved getPointedThing()

The new algorithm checks every node exactly once.
Now the point and normal vector of the collision is also returned in the
PointedThing (currently they are not used outside of the function).
Now the CNodeDefManager keeps the union of all possible nodeboxes, so
the raycast won't miss any nodes. Also if there are only small
nodeboxes, getPointedThing() is exceptionally fast.
Also adds unit test for VoxelLineIterator.

* Cleanup, code move

This commit moves getPointedThing() and
Client::getSelectedActiveObject() to ClientEnvironment.
The map nodes now can decide which neighbors they are connecting to
(MapNode::getNeighbors()).

src/mapnode.h

@@ -28,6 +28,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <vector>
 
 class INodeDefManager;
+class Map;
 
 /*
 	Naming scheme:
@@ -246,6 +247,13 @@ struct MapNode
 
 	void rotateAlongYAxis(INodeDefManager *nodemgr, Rotation rot);
 
+	/*!
+	 * Checks which neighbors does this node connect to.
+	 *
+	 * \param p coordinates of the node
+	 */
+	u8 getNeighbors(v3s16 p, Map *map);
+
 	/*
 		Gets list of node boxes (used for rendering (NDT_NODEBOX))
 	*/