#3 Sideways move overlaps pieces

[Quincunx55555]

I found that if I am trying to slide a piece under one
that has already landed, that I can move the piece
_through_ the established one; making the landed piece
like a "ghost piece" since it doesn't prevent the
sideways movement of the piece that is dropping next to
it. Let me know if you need more of an explanation.

[Federico Pelloni]

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I understood. You're right, I completely forgot to think
about this possibility when developing the move-left/-right
callbacks. I'll modify this as soon as I can, but I'll
probably need to rethink some quite big part of microtetris.