Stop keeping track of the vertical screen position when searching for
something.  If nothing is found, current_y doesn't change, and all the
incrementing/decrementing was a waste of time.  If something is found
and it is onscreen, it is easy to calculate the new current_y.  And if
something is found and it is offscreen, then current_y is irrelevant,
because we will be either centering the found occurrence (searching)
or putting it on the top or bottom line (bracket matching).

(The above does not take softwrapping into account, but neither did
the old code, so this doesn't introduce any new bugs.)

(Also, when the search wraps, and the viewport is away from head or
tail of the file, and the found occurrence is within the viewport,
then the incremented/decremented current_y would be way wrong, but
this didn't have any adverse effects as far as I could tell.  It
seems that current_y is irrelevant in most cases.)