The Invader itself occupies a 5 x 5 grid of blocks (25 bits). Internally,
the Invader is represented by a 3 x 5 grid (15 bits) because it
is also horizontally symmetric. The left side of the Invader simple
draws identical to the right.
If we iterate through all possible combinations of lighting bits
in a 3x5 grid, and eliminate no invaders on the basis of shifted
similarity, there are 32,768 (2 to the 15th power) unique Invaders.