Efficiently rendering point clouds of millions of points

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Two smart optimizations while rendering to efficiently draw clouds of millions of points:
- 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).
- Display lists.

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