patch 8.0.1136: W_WIDTH() is always the same

Problem:    W_WIDTH() is always the same.
Solution:   Expand the macro.
diff --git a/src/gui_mac.c b/src/gui_mac.c
index 8803487..bbb0857 100644
--- a/src/gui_mac.c
+++ b/src/gui_mac.c
@@ -1612,7 +1612,7 @@
     else			/* Bottom scrollbar */
     {
 	sb_info = sb;
-	page = W_WIDTH(curwin) - 5;
+	page = curwin->w_width - 5;
     }
 
     switch (partCode)