Rename menuoffTexture to tempShakeTexture

It turns out this texture is only used as a temporary texture to draw
the screen with an offset before rendering it to the output target.

I thought it was used for drawing the map menu animation, but that was
only true of `tempBuffer`, and is no longer true of the new render
system.

desktop_version/src/Graphics.h

@@ -308,7 +308,7 @@ public:
     bool notextoutline;
 
     SDL_Texture* gameTexture;
-    SDL_Texture* menuoffTexture;
+    SDL_Texture* tempShakeTexture;
     SDL_Texture* gameplayTexture;
     SDL_Texture* menuTexture;
     SDL_Texture* ghostTexture;