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/Make_morph.mak b/src/Make_morph.mak
index 87b7e45..a69eda0 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -39,6 +39,8 @@
 	dict.c							\
 	diff.c							\
 	digraph.c						\
+	drawline.c						\
+	drawscreen.c						\
 	edit.c							\
 	eval.c							\
 	evalbuffer.c						\