This commit introduces Raycast, a Lua user object, which can be
used to perform a raycast on the map. The ray is continuable, so one can
also get hidden nodes (for example to see trough glass).
* 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()).
