icon/neverball.png: revert from grayscale to RGBA

Setting the window icon via SDL requires creating an SDL_Surface
first, but for whatever reason SDL_Surface does not recognize
grayscale (with or without alpha) as a valid image format. SDL 1.2
doesn't even detect the error, but simply gives us an empty surface.

git-svn-id: https://s.snth.net/svn/neverball/trunk@3980 78b8d119-cf0a-0410-b17c-f493084dd1d7