main: Add an optional appstate param to main callback entry points.

This allows apps to maintain state data without using global variables.

Fixes #9377.

test/testaudio.c

@@ -1036,7 +1036,7 @@ static void WindowResized(const int newwinw, const int newwinh)
     state->window_h = newwinh;
 }
 
-int SDL_AppInit(int argc, char *argv[])
+int SDL_AppInit(void **appstate, int argc, char *argv[])
 {
     int i;
 
@@ -1094,7 +1094,7 @@ int SDL_AppInit(int argc, char *argv[])
 
 static SDL_bool saw_event = SDL_FALSE;
 
-int SDL_AppEvent(const SDL_Event *event)
+int SDL_AppEvent(void *appstate, const SDL_Event *event)
 {
     Thing *thing = NULL;
 
@@ -1214,7 +1214,7 @@ int SDL_AppEvent(const SDL_Event *event)
     return SDLTest_CommonEventMainCallbacks(state, event);
 }
 
-int SDL_AppIterate(void)
+int SDL_AppIterate(void *appstate)
 {
     if (app_ready_ticks == 0) {
         app_ready_ticks = SDL_GetTicks();
@@ -1232,7 +1232,7 @@ int SDL_AppIterate(void)
     return 0;  /* keep going. */
 }
 
-void SDL_AppQuit(void)
+void SDL_AppQuit(void *appstate)
 {
     while (things) {
         DestroyThing(things);  /* make sure all the audio devices are closed, etc. */