Efficiently rendering point clouds of millions of points

Three optimizations have been implemented in the rendering engine to allow drawing clouds of millions of points:

• Display lists.
• It uses octrees to avoid drawing parts of the point cloud out of the screen.
• For each visible octree node (a 3D cube), it decimates the points if the density of points per square pixel is above a given limit (See mrpt::global_settings::OCTREE_RENDER_MAX_DENSITY_POINTS_PER_SQPIXEL).