fixed triangulation bug

[elalish]

I extracted the problem polygon from #171, made a test and fixed it. Also did a little cleanup and ensuring the early geometric checks are being performed properly in debug mode.

In the process I found another bug, which I added a disabled test for so I can deal with it in the next PR. It comes up in Sponge4 with #171, but not on master, so figured I'd go ahead and merge this first. Again, this is a legit triangulation failure, not a problem with #171.

[codecov-commenter]

Codecov Report

Merging #180 (a37b060) into master (a08a9e7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #180   +/-   ##
=======================================
  Coverage   96.57%   96.57%           
=======================================
  Files          30       30           
  Lines        2630     2631    +1     
=======================================
+ Hits         2540     2541    +1     
  Misses         90       90           

Impacted Files	Coverage Δ
src/manifold/src/manifold.cpp	89.65% <ø> (ø)
src/polygon/src/polygon.cpp	98.51% <100.00%> (+<0.01%)	⬆️

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