Add Tessellate method that takes a new normal for the sweeping direction

Sometimes it's needed to override the sweeping direction to overcome edge cases.

Author: speps

LibTessDotNet/Sources/Tess.cs

@@ -705,7 +705,12 @@ public void Tessellate(WindingRule windingRule, ElementType elementType, int pol
 
         public void Tessellate(WindingRule windingRule, ElementType elementType, int polySize, CombineCallback combineCallback)
         {
-            _normal = Vec3.Zero;
+            Tessellate(windingRule, elementType, polySize, null, Vec3.Zero);
+        }
+
+        public void Tessellate(WindingRule windingRule, ElementType elementType, int polySize, CombineCallback combineCallback, Vec3 normal)
+        {
+            _normal = normal;
             _vertices = null;
             _elements = null;