More accurate view distance culling

[Ralith]

Currently, Graph::nearby_nodes only considers node centers. This causes chunks which fall within view distance, but which lie in nodes that do not, not to be rendered. While an exact solution requires potentially nontrivial hyperbolic geometry predicates, we could achieve a significantly closer approximation by checking distance to each vertex of the node and rendering chunks which lie on vertices that fall within view distance even if the center of their node does not. Alternatively, chunk bounding spheres could be used as discussed in #100.