Flush output before starting a potentially time-consuming

search operation. Without this, buffered data may be not
written or only partially written to the screen, so the
screen could look inconsistent while the search operation
is in progress.
Related to #533.

search.c

@@ -572,6 +572,7 @@ public POSITION next_unfiltered(POSITION pos)
  if (pos_in_header(pos))
  return (pos);
 
+ flush();
  n = hlist_find(&filter_anchor, pos);
  while (n != NULL && pos >= n->r.hl_startpos)
  {
@@ -594,6 +595,7 @@ public POSITION prev_unfiltered(POSITION pos)
  if (pos_in_header(pos))
  return (pos);
 
+ flush();
  n = hlist_find(&filter_anchor, pos);
  while (n != NULL && pos >= n->r.hl_startpos)
  {
@@ -1231,6 +1233,7 @@ static POSITION get_lastlinepos(POSITION pos, POSITION tpos, int sheight)
 {
  int nlines;
 
+ flush();
  for (nlines = 0;; nlines++)
  {
  POSITION npos = forw_line(pos);
@@ -1526,6 +1529,7 @@ static int search_range(POSITION pos, POSITION endpos, int search_type, int matc
  /* When the search wraps around, end at starting position. */
  if ((search_type & SRCH_WRAP) && endpos == NULL_POSITION)
  endpos = pos;
+ flush();
  for (;;)
  {
  /*