Update: drop-in multiplayer, proximity barrels, major graphics update

What's new:

• Drop-in multiplayer support: players are no longer automatically spawned on a level, they can enter by pressing jump.
• Barrels will now start shaking when players are near them and will explode at one point (unless the players leave its proximity).
• New spike graphics.
• New wall graphics on level 3.
• New weapon/projectile graphics: pistol, rocket launcher, rocket and grenade.
• New shard graphics: saw blades, exploding barrel, grenade.
• Picking up targets are now accompanied by an animated counter which shows the player's current target count in the player's color.
• All in-game fonts are now "Press Start 2P" by the amazing Cody "CodeMan38" Boisclair.
• New background image on all levels.
• Box and blob enemy spawns are now animated.
• Bugfix: activating a placed mine sometimes caused another mine in placement mode to explode, leaving the player in placement mode stuck.

Hotfix (2021-09-01 22:45): barrels could proximity-explode while in placement mode.