perf(column): keep track of number of lines that hold up the 'signcolumn'

Problem:  The entire marktree needs to be traversed each time a sign is
          removed from the sentinel line.
Solution: Remove sentinel line and instead keep track of the number of
          lines that hold up the 'signcolumn' in "max_count". Adjust this
          number for added/removed signs, and set it to 0 when the
          maximum number of signs on a line changes. Only when
          "max_count" is decremented to 0 due to sign removal do we need
          to check the entire buffer.

          Also replace "invalid_top" and "invalid_bot" with a map of
          invalid ranges, further reducing the number of lines to be
          checked.

          Also improve tree traversal when counting the number of signs.
          Instead of looping over the to be checked range and counting
          the overlap for each row, keep track of the overlap in an
          array and add this to the count.

src/nvim/option.c

@@ -6172,20 +6172,6 @@ bool fish_like_shell(void)
   return strstr(path_tail(p_sh), "fish") != NULL;
 }
 
-/// Return the number of requested sign columns, based on current
-/// buffer signs and on user configuration.
-int win_signcol_count(win_T *wp)
-{
-  if (wp->w_minscwidth <= SCL_NO) {
-    return 0;
-  }
-
-  int needed_signcols = buf_signcols(wp->w_buffer, wp->w_maxscwidth);
-  int ret = MAX(wp->w_minscwidth, MIN(wp->w_maxscwidth, needed_signcols));
-  assert(ret <= SIGN_SHOW_MAX);
-  return ret;
-}
-
 /// Get window or buffer local options
 dict_T *get_winbuf_options(const int bufopt)
   FUNC_ATTR_WARN_UNUSED_RESULT