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/api/vim.c

@@ -23,6 +23,7 @@
 #include "nvim/context.h"
 #include "nvim/decoration.h"
 #include "nvim/decoration_provider.h"
+#include "nvim/drawscreen.h"
 #include "nvim/edit.h"
 #include "nvim/eval.h"
 #include "nvim/eval/typval.h"
@@ -55,7 +56,6 @@
 #include "nvim/os/process.h"
 #include "nvim/popupmenu.h"
 #include "nvim/runtime.h"
-#include "nvim/screen.h"
 #include "nvim/state.h"
 #include "nvim/types.h"
 #include "nvim/ui.h"