Space Invaders fidelity bugs

[etosch]

• Right now in TB if a column is cleared, it doesn't confer any advantages. Enemies always move between the same horizontal endpoints. However, in real space invaders, when an end column is cleared, the enemies' range of motion increases by that column's width.

• According to the manual, the mothership sometimes goes L-R and sometimes R-L. Ours only goes L-R.