Make shock damage only affect frames.

What it says on the tin: shock damage from collisions only damages the square that was hit, and nearby frames.

The frame is what absorbs the shock of the impact, so it is what should be damaged. It doesn't make any sense to ram into a zombie and have your seatbelt explode.

Doesn't address the issue where collisions with zombies always think that (0, 0) collided with the monster, but that's a separate issue. Should help reign in some of the silliness with shock damage; more planned for later.