add example

example/main.cpp

@@ -1,4 +1,5 @@
 #include "layers/ui_layer/ui_layer.hpp"
+#include "layers/ui_layer_double/ui_layer_double.hpp"
 
 namespace example
 {
@@ -15,7 +16,8 @@ auto main(int argc, char* args[]) -> int
   example::app app{ "vertex_engine_sandbox" };
 
   app.push_layer(
-    new example::ui_layer{}
+    new example::ui_layer{},
+    new example::ui_layer_double{}
   );
   app.run();
 }