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

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

@@ -24,13 +24,8 @@ TEST_CASE("Empty Difference3D", "[Levelset Boolean]") {
 
 TEST_CASE("Empty Difference2D", "[Levelset Boolean]") {
   double height = 100, radius = 10;
-
-  csg::LinearExtrude my_lin_ext;
-  my_lin_ext.height = 100;
-  my_lin_ext.center = false;
-  my_lin_ext.scale = {1, 1};
-  my_lin_ext.group = csg::Union2D();
-
+  auto my_lin_ext = csg::LinearExtrude{
+      .height = 100, .center = false, .scale = {1, 1}, .group = csg::Union2D()};
   auto my_diff = csg::Difference2D();
 
   my_lin_ext.center = false;
@@ -47,4 +42,4 @@ TEST_CASE("Empty Difference2D", "[Levelset Boolean]") {
   CHECK_FALSE(0 < my_levelset(9, 0, 0));
 }
 
-} // namespace
+}