fix decimator

[pca006132]

#529 (comment)

1. DedupeEdge must dedupe the edge with the smallest index. Violating this property will cause 'halfedge_.size() % 6 == 0' is false: Not an even number of faces after sorting faces! .
2. CollapseTri and RemoveIfFolded can encounter collapsed edges, we should simply skip them.

Closes #529

[pca006132]

btw for 1, I verified this by inverting the arrays, but I did not commit it here.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (cc83e1b) 91.50% compared to head (baea544) 91.50%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   91.50%   91.50%           
=======================================
  Files          35       35           
  Lines        4520     4521    +1     
=======================================
+ Hits         4136     4137    +1     
  Misses        384      384           

Files Changed	Coverage Δ
src/manifold/src/edge_op.cpp	95.34% <100.00%> (+0.01%)	⬆️

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[elalish]

Thank you!