Fix overdraw bug in tower background

This could happen if you held down ACTION in the credits, looks like the
background doesn't keep up for some reason. That's another bug to fix,
but at least I can fix this overdraw.

desktop_version/src/Graphics.cpp

@@ -2379,6 +2379,7 @@ void Graphics::drawtowerspikes()
 
 void Graphics::drawtowerbackground()
 {
+    FillRect(backBuffer, 0x000000);
     SDL_BlitSurface(towerbuffer, NULL, towerbuffer_lerp, NULL);
     ScrollSurface(towerbuffer_lerp, 0, lerp(0, -map.bscroll));
     SDL_BlitSurface(towerbuffer_lerp,NULL, backBuffer,NULL);