Render cursor as a crosshair.

Gimp XCF included, some different styles in the layers.
Bail out of bullet instantiation if guy->cursor vector length is zero

i.e. Guy and cursor are at the same coords, no direction to aim in.
Crosshair XCF was updated.
Moved shoot, hit, constrain_velocity_xy and alive from Guy to Thing.

These can now be used for other Things like Baddies.

Also imported standard Collision::Util methods.
Initial version of Baddie, can be instantiated and moved.
Do bigger bullets help hit detection?
Bunch of stuff moved out to Thing. Added (rough) respawn method.
Added a queue of baddies and basic collision detection in move callback
vim junk > .gitignore