Revert "Use DefWindowProc for the initial SDL window proc"

This reverts commit afeeef9cf5638d65ebea019f87c77a8c3977a8b5.

src/video/windows/SDL_windowsevents.c

@@ -2056,12 +2056,18 @@ int SDL_RegisterApp(const char *name, Uint32 style, void *hInst)
     SDL_Instance = hInst ? hInst : GetModuleHandle(NULL);
 
     /* Register the application class */
-    SDL_zero(wcex);
     wcex.cbSize = sizeof(WNDCLASSEX);
+    wcex.hCursor = NULL;
+    wcex.hIcon = NULL;
+    wcex.hIconSm = NULL;
+    wcex.lpszMenuName = NULL;
     wcex.lpszClassName = SDL_Appname;
     wcex.style = SDL_Appstyle;
-    wcex.lpfnWndProc = DefWindowProc;
+    wcex.hbrBackground = NULL;
+    wcex.lpfnWndProc = WIN_WindowProc;
     wcex.hInstance = SDL_Instance;
+    wcex.cbClsExtra = 0;
+    wcex.cbWndExtra = 0;
 
 #if !defined(__XBOXONE__) && !defined(__XBOXSERIES__)
     hint = SDL_GetHint(SDL_HINT_WINDOWS_INTRESOURCE_ICON);