Decrease Point Cloud Extraction Memory Usage

[william-silversmith]

The geometry based approach is getting relatively close. This would prevent the need to decompress the image slices for a savings of 8x.

• Finish correctly matching contours. This requires improving the inside/outside polygon logic (use winding algorithm?).
• Figure out how to make matching fast (currently O(N^2)).

See the https://github.com/seung-lab/crackle/tree/wms_point_cloud_geometric branch.