SDL2: use SDL_UpdateTexture instead of locking/unlocking

despite documentation warnings that it is slow, it gives me 10 more FPS on iOS simulator…
make sure that stacked items have at least one charge
SDLVideo::cmake: update for opengl branch changes
Replaced deprecated loadNibNamed
Merge pull request #16 from canassa/fix-loadnibnamed-deprecated

Replaced deprecated loadNibNamed