Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes vim/vim#4943)
https://github.com/vim/vim/commit/7528d1f6b5422750eb778dfb550cfd0b0e540964

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>

* The userreg function can now return a list. If a list is returned, the
yankreg will be set to kMTLineWise, and each element in the list will
become a line in the yankreg.

* Use tv_get_string to get strings from typevals.

* call tv_free on the tv sent to typval_to_yankreg to properly unref the
  values.

vim-patch:8.1.2082: some files have a weird name to fit in 8.3 characters

Problem:    Some files have a weird name to fit in 8.3 characters.
Solution:   Use a nicer names.
https://github.com/vim/vim/commit/30e8e73506e4522ef4aebf7d525c0e6ffe8805fd

Replace grid.h in screen.h and screen.h in buffer.h with grid_defs.h

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

Problem:    Mode is not cleared when leaving Insert mode.
Solution:   Clear the mode when got_int is set. (Ozaki Kiichi, closes vim/vim#4270)
https://github.com/vim/vim/commit/abc7c7fc5a098374f5543a237e6c9dd918848b34

Problem:    Peeking and flushing output slows down execution.
Solution:   Do not update the mode message when global_busy is set.  Do not
            flush when only peeking for a character. (Ken Takata)
https://github.com/vim/vim/commit/cb574f415486adff645ce384979bfecf27f5be8c

Omit inchar() change: it breaks too many tests.

N/A patches for version.c:

vim-patch:8.2.5170: tiny issues

Problem:    Tiny issues.
Solution:   Tiny improvements.
https://github.com/vim/vim/commit/944cc9ceba8868acd238264d4a3894803c566b37

Work on https://github.com/neovim/neovim/issues/459

this works similar to <c-o> or <c-\><c-o> in insert mode