Add window ID

src/window.c

@@ -20,6 +20,7 @@ bool init_window(window_t *wnd, const char *title, u64 width, u64 height) {
     return false;
   }
 
+  wnd->id = SDL_GetWindowID(wnd->window);
   wnd->title = title;
   wnd->width = width;
   wnd->height = height;
@@ -38,7 +39,7 @@ void cleanup_window(window_t *wnd) {
     wnd->window = NULL;
   }
 
-  wnd->width = wnd->height = 0;
+  wnd->width = wnd->height = wnd->id = 0;
 }
 
 void set_colour(const window_t *wnd, colour_t colour) {