Remove designator aggregate intialization so code compiles with C++17

src/csg/tests/levelset/extrude.t.cpp

@@ -174,8 +174,11 @@ TEST_CASE("Linear extrude of a triangle with hole", "[Levelset Primitives]") {
   double ht = 100.0, outer = 100.0, inner = 80.0;
   double thick = outer - inner;
 
-  auto my_lin_ext = csg::LinearExtrude{
-      .height = ht, .center = true, .scale = {1, 1}, .group = csg::Union2D()};
+  csg::LinearExtrude my_lin_ext;
+  my_lin_ext.height = ht;
+  my_lin_ext.center = true;
+  my_lin_ext.scale = {1, 1};
+  my_lin_ext.group = csg::Union2D();
 
   // A 100 x 100 right triangle with a 80 x 80 hole formed by using a polygon
   // and extrude