used tileset sprite sheet for asteroids

src/states/State.h

@@ -40,6 +40,7 @@ struct StateContext {
     // backgroundTex is set once in `main.cpp` and passed to states via this context.
     // Prefer reading this field instead of relying on any `extern SDL_Texture*` globals.
     SDL_Texture* blocksTex = nullptr;
+    SDL_Texture* asteroidsTex = nullptr;
     SDL_Texture* scorePanelTex = nullptr;
     SDL_Texture* statisticsPanelTex = nullptr;
     SDL_Texture* nextPanelTex = nullptr;