The function place_the_cursor() assumes that the viewport is up to date,
i.e., that current is in range of edittop.  When uncutting or inserting,
however, place_the_cursor() gets called on the out-of-date viewport
first, and then a screen refresh is scheduled (which would put the
viewport up to date).  This is backwards: the refresh should come before
the cursor placement, and the only reason it works anyway is because the
cap on the number of chunks to move backward papers over the problem by
keeping current_y in screen range regardless.

Fix this properly by simply setting current_y to the bottom row of the
screen instead of calling place_the_cursor().  This value of current_y
is only ever used when in smooth scrolling mode and the insertion (or
paste) pushed the cursor offscreen.  In other situations, this value is
overridden when place_the_cursor() gets called after a screen refresh.

After that fix, the cap on the number of chunks to move backward is no
longer needed.