fix compose index error

[pca006132]

Fixes #425 (comment)
Also, for #429, it seems that the error is now:

Triangulation failed! Precision = 0.000444848
Error in file: /home/pca006132/code/manifold/src/polygon/src/polygon.cpp (39): 'condition' is false: Not Geometrically Valid! None of the skipped verts is valid.
polys.push_back({
    {27.2981834, -7.01095772},  //
    {27.4199429, -7.07648039},  //
    {27.2981834, -7.01095724},  //
    {27.7852192, -7.27304506},  //
    {27.5622101, -6.52031708},  //
});
array([
  [27.2981834, -7.01095772],
  [27.4199429, -7.07648039],
  [27.2981834, -7.01095724],
  [27.7852192, -7.27304506],
  [27.5622101, -6.52031708],
])

[pca006132]

Btw is there a way I can avoid GitHub from auto-closing the linked issue?

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1615b72) 89.73% compared to head (8f8fba6) 89.74%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #430   +/-   ##
=======================================
  Coverage   89.73%   89.74%           
=======================================
  Files          35       35           
  Lines        4200     4201    +1     
=======================================
+ Hits         3769     3770    +1     
  Misses        431      431           

Impacted Files	Coverage Δ
src/manifold/src/csg_tree.cpp	91.82% <100.00%> (+0.03%)	⬆️

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[elalish]

Btw is there a way I can avoid GitHub from auto-closing the linked issue?

Yes, just avoid the word "Fixes". Maybe something like "partially addresses". Also, that bug is pretty convoluted - nothing wrong with closing it and starting a more specific one.

[elalish]

Good find, thanks!