Health power-up and minor tweaks

We've started thinking about how to incentivise the player to climb up. For that, we implemented a health power-up. It's pretty obvious, but we just wanted to test how it feels. Right now, I'm not sure if power-ups alone make climbing up rewarding enough, so need to think about something else, too.

We also made a small tweak: now a wreck always falls into the column that the player was in when they shot the enemy down. This gives the player more precise control over where the wreck lands. The idea is to emphasise the puzzle aspect of the game. I don't think this change alone did much at this point, but it may start making sense when we start tweaking the enemies' flight patterns. Our idea is that filling up a row of wrecks should be a small puzzle to solve.

That's all for now. Cheers.