Support VSync callback

examples/PoolBox/application/src/application.cpp

@@ -7,6 +7,7 @@
 #include <FontWriter/font_writer.hpp>
 #include <PSX/Audio/CDDA.hpp>
 #include <PSX/Periphery/periphery.hpp>
+#include <PSX/System/callbacks.hpp>
 #include <stdio.hpp>
 
 using namespace JabyEngine;
@@ -155,6 +156,10 @@ namespace LoadingScene {
     static void run() {
         if(old_state_changer != state_changer) {
             printf("Loading new state...\n");
+
+            Callback::VSyncCallback::install([]() {
+                printf("Still loading...\n");
+            });
             
             update();
             GPU::swap_buffers_vsync(1);
@@ -165,6 +170,7 @@ namespace LoadingScene {
             state_changer.asset_load();
             old_state_changer = state_changer;
             CDDA::pop_play();
+            Callback::VSyncCallback::uninstall();
         }
 
         state_changer.main();