Count just enough lines for half page scrolling

[luukvbaal]

Problem: Calculating line height for unnecessary amount of lines with
half-page scrolling.
Solution: Replace "limit_winheight" argument with higher resolution
"max" argument to which to limit the calculated line height
in plines_m_win() to.

Fix #14650

[luukvbaal]

Could also add this cheap check that determines whether calling plines_m_win() is even necessary to begin with, although that only works if the window has no folds:

@@ -3219,7 +3219,8 @@ pagescroll(int dir, long count, int half)

        int curscount = count;
        // Adjust count so as to not reveal end of buffer lines.
-       if (dir == FORWARD)
+       if (dir == FORWARD && (hasAnyFolding(curwin)
+                   || curwin->w_topline + count > buflen - curwin->w_height))
        {
            int n = plines_correct_topline(curwin, curwin->w_topline, FALSE);
            if (n - count < curwin->w_height && curwin->w_topline < buflen)

[zhscn]

I just applied this PR and the CPU usage goes down to 3.5%. 👍

[chrisbra]

thanks!