triangulator bugs related to #528

[pca006132]

Things from #529. Bugs:

Error message for triangulation:
Error in file: /home/pca006132/code/manifold/src/polygon/src/polygon.cpp (158): 'std::all_of(triangles.begin(), triangles.end(), [&vertPos, precision](const glm::ivec3 &tri) { return CCW(vertPos[tri[0]], vertPos[tri[1]], vertPos[tri[2]], precision) >= 0; })' is false: triangulation is not entirely CCW!
------------
Final polygon:
polys.push_back({
    {2.09305096, 5.86389685},  //
    {2.06142592, 5.85550117},  //
    {1.93763256, 5.4309001},  //
});
polys.push_back({
    {1.84193516, 5.90364313},  //
    {1.93763256, 5.4309001},  //
    {1.84193516, 5.90364313},  //
    {1.93763256, 5.4309001},  //
    {1.9408412, 5.90364313},  //
});
show(array([
  [2.09305096, 5.86389685],
  [2.06142592, 5.85550117],
  [1.93763256, 5.4309001],
]))
show(array([
  [1.84193516, 5.90364313],
  [1.93763256, 5.4309001],
  [1.84193516, 5.90364313],
  [1.93763256, 5.4309001],
  [1.9408412, 5.90364313],
]))
------------
Erroneous triangulation:
show(array([[1.8419352, 5.9036431], [1.9376326, 5.4309001], [1.9408412, 5.9036431]]))
show(array([[1.9408412, 5.9036431], [1.8419352, 5.9036431], [1.9376326, 5.4309001]]))
show(array([[1.8419352, 5.9036431], [1.9408412, 5.9036431], [1.9376326, 5.4309001]]))
^ not CCW
show(array([[2.0930510, 5.8638968], [2.0614259, 5.8555012], [1.9376326, 5.4309001]]))

Error in file: /home/pca006132/code/manifold/src/polygon/src/polygon.cpp (103): 'std::distance(forward.begin(), end) == n_edges' is false: Half of halfedges should be forward.
polys.push_back({
    {20.1267338, 2.00670671},  //
    {20.12673, 2.00670576},  //
    {19.7702141, 1.63188827},  //
    {19.7702141, 1.63188827},  //
    {19.7702141, 1.63188827},  //
    {19.5752983, 1.81728792},  //
    {19.5623493, 1.81283963},  //
    {19.7630997, 1.54846776},  //
    {19.7630997, 1.54846776},  //
});
polys.push_back({
    {19.7702141, 1.63188827},  //
    {19.898922, 1.92845321},  //
    {19.5752983, 1.81728792},  //
});
polys.push_back({
    {19.7630997, 1.54846776},  //
    {19.7630997, 1.54846776},  //
    {19.7630997, 1.54846776},  //
});
array([
  [20.1267338, 2.00670671],
  [20.12673, 2.00670576],
  [19.7702141, 1.63188827],
  [19.7702141, 1.63188827],
  [19.7702141, 1.63188827],
  [19.5752983, 1.81728792],
  [19.5623493, 1.81283963],
  [19.7630997, 1.54846776],
  [19.7630997, 1.54846776],
])
array([
  [19.7702141, 1.63188827],
  [19.898922, 1.92845321],
  [19.5752983, 1.81728792],
])
array([
  [19.7630997, 1.54846776],
  [19.7630997, 1.54846776],
  [19.7630997, 1.54846776],
])