The number of lines to scroll is: the y position of the start of the
current line, plus the extra lines that this line occupies, plus the
extra lines that the next line occupies, plus one, minus the y position
of the last window line.

The y position of the start of the current line is current_y -
xplustabs() / COLS, the extra lines are strlenpt(data) / COLS,
and the y position of the last window line is editwinrows - 1.

Note that we first compute the amount to scroll before actually moving
to the next line, because we need the current value of current_x, not
the one that it will have in the next line.  The placewewant value is
not good either, because it might be beyond where we actually are.

This fixes https://savannah.gnu.org/bugs/?47665.