Reset MaxHP at start of level
Make orb enemy easier to kill (lower HP)
Add 4th joy button for smashing

Joy3 was already used to access the menu. I added joy4 and swapped it so
that joy3 is for smashing and joy4 is for the menu.
Update Level 2

I also fixed some quirks with the block explosions / wall smashing.
Update sprite for gas can / tweak ammo pick-up behavior
Update Level 2
Raise ammo/gas limit