Scale back depth of game.

It would be too boring to dig more than 50 kms deep.
That's way too many tiles to dig.

The focus should be on the challenge of getting 50 kms deep,
rather than getting there fast.

src/app.rs

@@ -57,6 +57,7 @@ impl App {
 
     fn do_update(&mut self) {
         let tp = self.game.camera_offset.tile_pos();
+        self.game.camera_offset.y += 33;
         imm_dbg!(tp);
         imm_dbg!(tp.to_chunk_and_local());
     }