patch 8.2.4644: redrawing too often when 'relativenumber' is set

Problem:    Redrawing too often when 'relativenumber' is set.
Solution:   Only redraw when the cursor line changed. (Lewis Russell,
            closes #10040)
diff --git a/src/structs.h b/src/structs.h
index a153336..192693b 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -3465,6 +3465,9 @@
     colnr_T	w_old_visual_col;   // last known start of visual part
     colnr_T	w_old_curswant;	    // last known value of Curswant
 
+    linenr_T    w_last_cursor_lnum_rnu;  // cursor lnum when 'rnu' was last
+					 // redrawn
+
     lcs_chars_T	w_lcs_chars;	    // 'listchars' characters
 
     /*