patch 8.1.2057: the screen.c file is much too big

Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes #4943)
diff --git a/src/proto.h b/src/proto.h
index 842d891..efedc0e 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -73,6 +73,8 @@
 # include "dict.pro"
 # include "diff.pro"
 # include "digraph.pro"
+# include "drawline.pro"
+# include "drawscreen.pro"
 # include "edit.pro"
 # include "eval.pro"
 # include "evalbuffer.pro"