Implement and Direction2, Direction3, Vector2 classes

Directions represent euler angles (2D and 3D resp.) and automatically
handle e.g. wrapping yaw to fit [0; 2*pi] range

Use Direction for Heli and Bullet constructor
Change resolution to 320x200

This is actually native Strike resoluton; I was misleaded by dosbox
using 2x scaling
Fix firing

Implement gun offset and fire direction granularity (which corresponds
to 24 helicopter direction sprites)
Move chopper to the left for weapons testing
Implement g-force affecting bullets