new background for main screen

src/states/State.h

@@ -39,6 +39,9 @@ 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* mainScreenTex = nullptr;
+    int mainScreenW = 0;
+    int mainScreenH = 0;
 
     // Audio / SFX - forward declared types in main
     // Pointers to booleans/flags used by multiple states