When a multi-column character straddles a chunk boundary, and the
preferred column (placewewant) for the cursor is zero, cheat: show
the cursor not where the character starts but on the beginning of
the next row.  This makes the cursor move smoothly in the leftmost
column of the screen when using <Up> and <Down> and such, instead
of jumping around.

In this way the scrolling logic won't get confused and the screen
will scroll properly when stepping beyond the top or bottom row.

This fixes https://savannah.gnu.org/bugs/?50687.