vim-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 vim/vim#4943)
7528d1f6b5

This is an approximation vim-patch 8.1.2057. Applying the patch directly
isn't feasible since our version of screen.c has diverged too much,
however we still introduce drawscreen.c and drawline.c:
- screen.c is now a much smaller file used for low level screen functions
- drawline.c contains everything needed for win_line()
- drawscreen.c contains everything needed for update_screen()

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

src/nvim/diff.c

@@ -20,6 +20,7 @@
 #include "nvim/charset.h"
 #include "nvim/cursor.h"
 #include "nvim/diff.h"
+#include "nvim/drawscreen.h"
 #include "nvim/eval.h"
 #include "nvim/ex_cmds.h"
 #include "nvim/ex_docmd.h"
@@ -36,7 +37,6 @@
 #include "nvim/os/os.h"
 #include "nvim/os/shell.h"
 #include "nvim/path.h"
-#include "nvim/screen.h"
 #include "nvim/strings.h"
 #include "nvim/ui.h"
 #include "nvim/undo.h"