Allow block placement when a the placer can't see the player blocking

This change makes sure that players  not seen by a player placing a block, can't block said placement. I've discussed what the most appropriate place for this change would be, but this is where we ended up. In case performance is a concern a specific case of `a(AABB, EntityPlayer)` (Player places block) can be split out, however this conflicts with the minimal-diff policy, since it requires changes in mayPlace.