Flip triangles in csg_tree

[pca006132]

Although we already have related logic in impl.cpp, the compose code will skip performing transformation on the original mesh, and apply transformation on the combined mesh directly, skipping the previous fix. The logic is now added to the compose function to fix the bug without much performance penalty. This also factors out some functions used in both impl.cpp and csg_tree.cpp (although we should probably give it a better name).

Fixes #403

[pca006132]

@elalish is there a way to check if triangle orientations are correct? I think we should add this to our tests as this bug occurred at least twice.

[codecov]

Codecov Report

Patch coverage: 94.87% and project coverage change: -0.16 ⚠️

Comparison is base (f25d960) 86.04% compared to head (f4e1686) 85.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #404      +/-   ##
==========================================
- Coverage   86.04%   85.88%   -0.16%     
==========================================
  Files          36       37       +1     
  Lines        4485     4500      +15     
==========================================
+ Hits         3859     3865       +6     
- Misses        626      635       +9     

Impacted Files	Coverage Δ
src/manifold/src/impl.cpp	94.97% <ø> (-0.29%)	⬇️
src/manifold/src/mesh_fixes.h	91.30% <91.30%> (ø)
src/manifold/src/csg_tree.cpp	92.56% <100.00%> (+0.22%)	⬆️

... and 1 file with indirect coverage changes

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[elalish]

Thanks! As for tests, the best check of triangle orientation is the sign of the volume. For compose, you might want to decompose and check each volume's sign, or make sure the total is correct. There's also our testing hook, MatchesTriNormals(), but that's more of an internal consistency check.