Attack animation

Small update!

The attack now has an animation.
You have a small duration of invincibility so going for an attack is less punishing.
Your attack hits zombies over a short duration instead of for only a single frame.

Other improvements:

Improved camera for less jitteriness and motion sickness.
Camera has become better at keeping the player and the zombies on screen.
Jump is more responsive by shortening airtime of player. It's easier to nail a jump over the zombies, and you're back in the game faster.

Fixes:

Fixed an erroneously placed spawn point. The 4 green portals now work correctly.