the multi_window example breaks when using PistonWindow<Sdl2Window>

[lspitzner]

The underlying issue is that Sdl2Window conflates singleton initialization and window instance initialization when it calls sdl2::init in its new method.

I am not sure if this is a) not supposed to work b) supposed to work, but with different code, meaning that the example is chosen badly (issue for piston_examples) c) a bug in sdl2_window d) a bug in piston_window e) something else; so I am reporting this here.

(Also it seems non-trivial with the current interface(s) to factor out the sdl2 initialization - in my best attempt I end up with several copy-pastas from different builders and it still does not work completely (yet - I'll keep trying).)

piston = "0.27.0"
piston_window = "0.58.0"