Problem: The screen.c file is much too big.
Solution: Split it in three parts. (Yegappan Lakshmanan, closesvim/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>
