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/grid.h

@@ -19,6 +19,9 @@ EXTERN ScreenGrid default_grid INIT(= SCREEN_GRID_INIT);
 
 #define DEFAULT_GRID_HANDLE 1  // handle for the default_grid
 
+/// While resizing the screen this flag is set.
+EXTERN bool resizing_screen INIT(= 0);
+
 EXTERN schar_T *linebuf_char INIT(= NULL);
 EXTERN sattr_T *linebuf_attr INIT(= NULL);