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>

- Remove autocmd.h from fileio.h
- Remove normal.h from main.h
- Move bufinfo_T from undo_defs.h to undo.c

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

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
https://github.com/vim/vim/commit/a80faa8930ed5a554beeb2727762538873135e83

Problem:    Code has more indent than needed.
Solution:   Use continue and return statements. (closes vim/vim#10824)
https://github.com/vim/vim/commit/101d57b34b72f4fbc7df1b6edfd64c64a6be14fc

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

Problem:    Invalid memory access in diff mode with "dp" and undo.
Solution:   Make sure the line number does not go below one.
https://github.com/vim/vim/commit/4e677b9c40ccbc5f090971b31dc2fe07bf05541d

Problem:    Accessing freed memory with diff put.
Solution:   Bail out when diff pointer is no longer valid.
https://github.com/vim/vim/commit/c5274dd12224421f2430b30c53b881b9403d649e

Problem:    Invalid memory access after diff buffer manipulations.
Solution:   Use zero offset when change removes all lines in a diff block.
https://github.com/vim/vim/commit/c101abff4c6756db4f5e740fde289decb9452efa