Avoid T-junction

[finscn]

Avoid T-junction.

Ensure all edges are only intersect at the vertices.

Solve the issue ： #74

[mourner]

Unfortunately many tests fail with this change.

[finscn]

@mourner , In fact , I'm very weak at "Computer Graphics" , I give up PR for this issue.

But I hope you could know there is a serious bug at current earcut. And I hope you could fix it , Because you're an expert.

I'm just a fat boy ,and I need earcut for my game.

Thank you very much.