Optimize spatial queries used by chunk/entity rendering

[Ralith]

To compute the set of chunks/entities that need to be rendered, we traverse the portion of the graph overlapping with a sphere around the viewpoint. This is empirically a major CPU time sink. Available optimizations include:

• Avoid redundant per-frame traversals
• Skip portions of the graph outside the view frustum
• Precompute node-to-node transform matrices and edges to traverse using a secondary graph. See also remaining work in Optimize chunk transform streaming #55.

See also #73 for refactoring of related code.