The sprites in Airstrike are organized into different
sprite_group_t's, depending on their purpose. Short summary follows,
in the same order as the sprites are drawn.

bg_mech_group: 

Sprites that are part of the scenery, but which does
not interact with other sprites of their kind. Example: trees. This
is done to save cpu on collision detection and handling. This group
interacts with mech_group. Contains only sprite of type mech_sprite_t.

mech_group:

This is where the main action takes place. Full mechanics between all
sprites in this group. Contains only sprite of type mech_sprite_t.
Interacted with itself, bg_mech_group and bullet_group.

bullet_group:

No advanced mechanics, and no self-interaction. Sprites in this
group are simply of type sprite_t. Interacts with mech_group.

effects_group:

Sprites of type sprite_t. Does not interact (but should, perhaps ?).
Typically explosions and smoke.

foreground_group:

Sprites of type sprite_t. Does not interact. Sprites here are
of course guaranteed to be drawn on top of all other kinds. For
clouds etc.

ui_group:

obsolete, the ui should be handled in other ways now.
