Triangulator robustness

[elalish]

So, this doesn't actually solve #148, but it does fix part of it. I found my triangulator was failing on certain completely colinear polygons. The fix is fairly simple, since they just need to be identified - the simple monotone triangulator already works for them since any triangulation is admissible (all possible triangles are degenerate).

In the process of debugging I realized I didn't have as many intermediate checks operating in our tests as I had planned. I've enabled them now, which help the tests to catch more errors at the cost of a little slowdown (mostly on Sponge4, since most of its time is in triangulation). I also found and fixed a couple of bugs where I was accidentally recalculating normals instead of passing them along.

I've added the test from #148 as Disabled, since it still doesn't pass. Currently the triangulator fails on a simple polygon that is in fact ε-overlapped. I believe the issue is that the precision of the new intersection verts is worse than our precision calculation since nearly-parallel intersections can drive the error up to the size of the triangle overlaps. I'm not sure the best way to handle that - driving up the overall precision band with each operation will quickly make solutions impractical.

However, the good news is that I believe I now have a fairly simple way to create manifold (though geometrically invalid) triangulations from overlapped polygons. This should allow us to make all the triangulator exceptions optional, which was the biggest thing blocking #141. This may "solve" #148 too; the main question is whether small overlaps remain small or grow unacceptably large.

[codecov-commenter]

Codecov Report

Merging #154 (e63e9b3) into master (6247784) will increase coverage by 0.08%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   93.72%   93.81%   +0.08%     
==========================================
  Files          32       32              
  Lines        2838     2861      +23     
==========================================
+ Hits         2660     2684      +24     
+ Misses        178      177       -1     

Impacted Files	Coverage Δ
manifold/src/face_op.cpp	93.97% <ø> (-0.08%)	⬇️
polygon/src/polygon.cpp	88.07% <84.61%> (+0.47%)	⬆️
manifold/src/edge_op.cpp	99.21% <100.00%> (+0.02%)	⬆️
manifold/src/smoothing.cpp	96.17% <100.00%> (+0.03%)	⬆️
manifold/src/sort.cpp	100.00% <100.00%> (ø)
samples/src/rounded_frame.cpp	100.00% <100.00%> (ø)

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