Invoke finalizers on garbage collection

[airstruck]

Functions that create something that needs to be destroyed later should probably associate the finalizer with ffi.gc. For example:

function sdl.createTextureFromSurface (...)
    return ffi.gc(sdl.C.SDL_CreateTextureFromSurface(...), sdl.C.SDL_DestroyTexture)
end

This might cause unexpected behavior in certain cases (for example a cursor could get GC'd while still in use), but overall would probably be worth it. I'd be willing to make a PR if there's any interest in this.