Fix T51733: 3d print toolbox checks report false positives.

Colinear vertices in a same face would not be handled correctly.

source/blender/bmesh/intern/bmesh_queries.c

@@ -1515,16 +1515,30 @@ float BM_loop_calc_face_angle(const BMLoop *l)
  */
 void BM_loop_calc_face_normal(const BMLoop *l, float r_normal[3])
 {
-	if (normal_tri_v3(r_normal,
+#define FEPSILON 1e-5f
-	                  l->prev->v->co,
+
-	                  l->v->co,
+	/* Note: we cannot use result of normal_tri_v3 here to detect colinear vectors (vertex on a straight line)
-	                  l->next->v->co) != 0.0f)
+	 * from zero value, because it does not normalize both vectors before making crossproduct.
-	{
+	 * Instead of adding two costly normalize computations, just check ourselves for colinear case. */
-		/* pass */
+	/* Note: FEPSILON might need some finer tweaking at some point? Seems to be working OK for now though. */
+	float v1[3], v2[3], v_tmp[3];
+	sub_v3_v3v3(v1, l->prev->v->co, l->v->co);
+	sub_v3_v3v3(v2, l->next->v->co, l->v->co);
+
+	const float fac = (v2[0] == 0.0f) ? ((v2[1] == 0.0f) ? ((v2[2] == 0.0f) ? 0.0f : v1[2] / v2[2]) : v1[1] / v2[1]) : v1[0] / v2[0];
+
+	mul_v3_v3fl(v_tmp, v2, fac);
+	sub_v3_v3(v_tmp, v1);
+	if (fac != 0.0f && !is_zero_v3(v1) && len_manhattan_v3(v_tmp) > FEPSILON) {
+		/* Not co-linear, we can compute crossproduct and normalize it into normal. */
+		cross_v3_v3v3(r_normal, v1, v2);
+		normalize_v3(r_normal);
 	}
 	else {
 		copy_v3_v3(r_normal, l->f->no);
 	}
+
+#undef FEPSILON
 }
 
 /**