game: Update shields to use shield group buffers for drawing
game: Update shields to use shield group buffers for drawing

But keep the old code for creating shield game entities and then drawing
them as entities.  That's for backwards compatibility with the original.

TODO: need to remove that, have the computer version support tile-based
shields too.

Game loop time and data size unchanged.

eecsninja/invaders,eecsninja/invaders