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/mouse.c

@@ -8,6 +8,7 @@
 #include "nvim/charset.h"
 #include "nvim/cursor.h"
 #include "nvim/diff.h"
+#include "nvim/drawscreen.h"
 #include "nvim/fold.h"
 #include "nvim/grid.h"
 #include "nvim/memline.h"
@@ -15,7 +16,6 @@
 #include "nvim/move.h"
 #include "nvim/os_unix.h"
 #include "nvim/plines.h"
-#include "nvim/screen.h"
 #include "nvim/state.h"
 #include "nvim/strings.h"
 #include "nvim/syntax.h"