Implement painter's algorithm or similarly optimize z-render

[dankamongmen]

Currently, our rendering path is left-to-right, top-to-down, cell at a time. That might be changed by other issues (#155 etc.), but it's still O(p) on p planes per-cell. That's dumb. Use the painter's algorithm or something to instead descend plane-by-plane, looking only at the values that are exposed. This is of course still O(pc) on c cells, but typically it ought have much better performance.