Fix block shifting logic.

Most of these changes are from February, but there was a major bug I couldn't
figure out at the time.  Now I have. :)
Raise player box higher in z-axis.

When we got to the left-most column, the player was underneath the playing
board.  We've only been displayed as on top the rest by chance.
Add additional player movement debugging prints.

It's a bit disorienting to look at the place we were *previously* at, instead
of where we just moved.
Add off-screen column once at initialization.

Since we're no longer attempting to move the block objects themselves, but are
just transitioning an attribute, we only need to create the off-screen column
once, not on every shift.

This also solves a bug with the 0th column always appearing white, even after
visitation.