Establish VSyncCallback rendering

examples/PoolBox/application/src/application.cpp

@@ -248,17 +248,31 @@ namespace LoadingScene {
         FontWriter::new_font_writer.render();
     }
 
+    // TODO: No good name
+    static void test() {
+        const auto triangle1 = Make::POLY_F3({
+            Make::Vertex(0, 0),
+            Make::Vertex(128, 128),
+            Make::Vertex(0, 128)}, 
+            GPU::Color24::Blue());
+
+        GPU::swap_buffers(false);
+        GPU::render(triangle1);
+    }
+
     static void run() {
         if(Shared::load_test || old_state_changer != state_changer) {            
             update();
-            GPU::swap_buffers_vsync(1);
             render();
-            GPU::swap_buffers_vsync(1);
+
+            GPU::set_vsync_callback(test);
 
             cd_player.push();
             state_changer.asset_load();
             old_state_changer = state_changer;
             cd_player.pop();
+
+            GPU::set_vsync_callback(nullptr);
         }
 
         state_changer.main();