Revert to previous calculation for tube normals

Browser caching played a tricked me into thinking the new
approach looks as good as the old one.

src/gfx/geom-builders.js

@@ -115,8 +115,10 @@ function TubeProfile(points, num, strength) {
 
   var normal = vec3.create();
   for (var i = 0; i < this._arcs; ++i) {
-    normal[0] = this._verts[3 * i + 0];
-    normal[1] = this._verts[3 * i + 1];
+    var i_prev = i === 0 ? this._arcs - 1 : i - 1;
+    var i_next = i === this._arcs - 1 ? 0 : i + 1;
+    normal[0] = this._verts[3 * i_next + 1] - this._verts[3 * i_prev + 1];
+    normal[1] = this._verts[3 * i_prev] - this._verts[3 * i_next];
     vec3.normalize(normal, normal);
     this._normals[3 * i] = normal[0];
     this._normals[3 * i + 1] = normal[1];