graphics: Add endgame screen graphics.

Freedoom Phase 1 needs endgame screens for the ends of episodes
2, 3 and 4. Add some basic screens that are essentially just
screenshots from level 8 of each episode. Set END0-END6 to be the
dummy empty graphic so that the game doesn't crash after E3.

This (for the time being) fixes the most egregious problems in #32.