Antichess insufficient material with opposite-coloured bishops

[johndoknjas]

Closes lichess-org/lila#12966.

If side A has a bishop on colour complex X, and side B only has bishop(s) (and only on colour complex Y), then after ruling out certain stalemate constructions, we can conclude it's insufficient material for side A. I.e.:

• If side A has no pawns, stalemate is impossible.
• Or if side A only has one pawn (that's not a knight pawn that can allow the bishop to be stalemated in the corner, e.g.), and side B only has one bishop (preventing the pawn from changing files via a capture), stalemates are also impossible.