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/drawline.pro b/src/proto/drawline.pro
new file mode 100644
index 0000000..99163ac
--- /dev/null
+++ b/src/proto/drawline.pro
@@ -0,0 +1,3 @@
+/* drawline.c */
+int win_line(win_T *wp, linenr_T lnum, int startrow, int endrow, int nochange, int number_only);
+/* vim: set ft=c : */