Fix T91889 Exact boolean sometimes drops triangles.

The problem is that the fast triangulator (based on polyfill)
sometimes makes degenerate triangles. Commit 8115f0c5bd91f had
a check for degenerate triangles but it wasn't thorough enough.
This commit uses a more thorough (and pessimistic) test for
degenerate triangles, using the exact triangulator in those cases.

source/blender/blenlib/intern/mesh_intersect.cc

@@ -2225,8 +2225,7 @@ static bool face_is_degenerate(const Face *f)
   return false;
 }
 
-/** Fast check for degenerate tris. Only tests for when verts are identical,
- * not cases where there are zero-length edges. */
+/** Fast check for degenerate tris. It is OK if it returns true for nearly degenerate triangles. */
 static bool any_degenerate_tris_fast(const Array<Face *> triangulation)
 {
   for (const Face *f : triangulation) {
@@ -2236,6 +2235,23 @@ static bool any_degenerate_tris_fast(const Array<Face *> triangulation)
     if (v0 == v1 || v0 == v2 || v1 == v2) {
       return true;
     }
+    double3 da = v2->co - v0->co;
+    double3 db = v2->co - v1->co;
+    double da_length_squared = da.length_squared();
+    double db_length_squared = db.length_squared();
+    if (da_length_squared == 0.0 || db_length_squared == 0.0) {
+      return true;
+    }
+    /* |da x db| = |da| |db| sin t, where t is angle between them.
+     * The triangle is almost degenerate if sin t is almost 0.
+     * sin^2 t = |da x db|^2 / (|da|^2 |db|^2)
+     */
+    double3 dab = double3::cross_high_precision(da, db);
+    double dab_length_squared = dab.length_squared();
+    double sin_squared_t = dab_length_squared / (da_length_squared * db_length_squared);
+    if (sin_squared_t < 1e-8) {
+      return true;
+    }
   }
   return false;
 }