Dont create zero-length polygon edges

Work related to #55, this attempts to investigate and solve the problem of polygons being created with zero-length edges during BSP tree construction. 

Such polygons are dangerous, mathematically, to further operations and should not exist, or must be handled where computationally significant. 

[[CITATION OF DISCOVERY](https://github.com/sshirokov/csgtool/pull/55#issuecomment-43989448)]