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/raycast.h

@@ -0,0 +1,38 @@
+/*
+Minetest
+Copyright (C) 2016 juhdanad, Daniel Juhasz <juhdanad@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#ifndef SRC_RAYCAST_H_
+#define SRC_RAYCAST_H_
+
+
+/*!
+ * Checks if a line and a box intersects.
+ * @param[in]  box              box to test collision
+ * @param[in]  start            starting point of the line
+ * @param[in]  dir              direction and length of the line
+ * @param[out] collision_point  first point of the collision
+ * @param[out] collision_normal normal vector at the collision, points
+ * outwards of the surface. If start is in the box, zero vector.
+ * @returns true if a collision point was found
+ */
+bool boxLineCollision(const aabb3f &box, const v3f &start, const v3f &dir,
+	v3f *collision_point, v3s16 *collision_normal);
+
+
+#endif /* SRC_RAYCAST_H_ */