Enemy + bullet collisions

[pserwylo]

Things it does correctly:

• Bullet + enemy both removed if hit by a bullet.
• Fixes rendering of background map.
• Centres tank in map at start of game.
• Removes bullets when moving beyond the edge of the world.

Things that need improvement:

• Simple bounding box for enemies, and circle for bullets. Should be more nuanced with the shape of things colliding.
• No notion of damage or health - one bullet means enemy is dead.
• No feedback (explosions, particles, etc).

[pserwylo]

Oh, this can probably be ignored. Just noticed the MG Extended Collision package, and am refactoring this to use that package now.

[pserwylo]

Updated this to use the collision library. A lot better than the old implementation I think (though it is only ever going to work with simple shapes like boxes, circles, and ellipses. If more complex shapes are needed, we can probably worry about it later.