Avoid degenerate triangles when snapping to mesh.

This can cause memory exhaustion when exporting to mesh with small
(but not excessively small) chord tolerance.

src/mesh.cpp

@@ -601,6 +601,10 @@ void SKdNode::SnapToVertex(Vector v, SMesh *extras) {
             if(tr->b.Equals(v)) { tr->b = v; continue; }
             if(tr->c.Equals(v)) { tr->c = v; continue; }
 
+            if(tr->IsDegenerate()) {
+                continue;
+            }
+
             if(v.OnLineSegment(tr->a, tr->b)) {
                 STriangle nt = STriangle::From(tr->meta, tr->a, v, tr->c);
                 extras->AddTriangle(&nt);
@@ -632,6 +636,9 @@ void SKdNode::SnapToMesh(SMesh *m) {
     int i, j, k;
     for(i = 0; i < m->l.n; i++) {
         STriangle *tr = &(m->l.elem[i]);
+        if(tr->IsDegenerate()) {
+            continue;
+        }
         for(j = 0; j < 3; j++) {
             Vector v = tr->vertices[j];
 
@@ -1117,10 +1124,7 @@ void SMesh::PrecomputeTransparency() {
 
 void SMesh::RemoveDegenerateTriangles() {
     for(auto &tr : l) {
-        bool isDegenerate = tr.a.OnLineSegment(tr.b, tr.c) ||
+        tr.tag = (int)tr.IsDegenerate();
-                            tr.b.OnLineSegment(tr.a, tr.c) ||
-                            tr.c.OnLineSegment(tr.a, tr.b);
-        tr.tag = (int)isDegenerate;
     }
     l.RemoveTagged();
 }

src/polygon.cpp

@@ -87,6 +87,12 @@ double STriangle::SignedVolume() const {
     return a.Dot(b.Cross(c)) / 6.0;
 }
 
+bool STriangle::IsDegenerate() const {
+    return a.OnLineSegment(b, c) ||
+           b.OnLineSegment(a, c) ||
+           c.OnLineSegment(a, b);
+}
+
 void STriangle::FlipNormal() {
     swap(a, b);
     swap(an, bn);

src/polygon.h

@@ -191,6 +191,7 @@ public:
     bool Raytrace(const Vector &rayPoint, const Vector &rayDir,
                   double *t, Vector *inters) const;
     double SignedVolume() const;
+    bool IsDegenerate() const;
 };
 
 class SBsp2 {