Cocoa_ShowMessageBox() should call Cocoa_RegisterApp().

Otherwise, message boxes can fail if SDL_Init(SDL_INIT_VIDEO) wasn't completed.

src/video/cocoa/SDL_cocoaevents.m

@@ -156,6 +156,7 @@ CreateApplicationMenus(void)
 void
 Cocoa_RegisterApp(void)
 {
+    /* This can get called more than once! Be careful what you initialize! */
     ProcessSerialNumber psn;
     NSAutoreleasePool *pool;
 

src/video/cocoa/SDL_cocoamessagebox.m

@@ -37,6 +37,8 @@
 int
 Cocoa_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *buttonid)
 {
+    Cocoa_RegisterApp();
+
     NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
 
     NSAlert* alert = [[NSAlert alloc] init];