Controls tank shell behavior during flight and explosion.

Bit(s)	Meaning
0-2	Speed (1-4): horizontal pixels per frame
5	TANK_SHELL_BIT_EXPLODING: explosion animation active
6	Orientation (same as SLOT_BIT_ORIENTATION)
7	TANK_SHELL_BIT_FLYING: shell in flight

Speed is pseudo-random (1-4), set when shell is fired at tank_fire_shell_entry.