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/drawscreen.h

@@ -0,0 +1,25 @@
+#ifndef NVIM_DRAWSCREEN_H
+#define NVIM_DRAWSCREEN_H
+
+#include "nvim/drawline.h"
+
+/// flags for update_screen()
+/// The higher the value, the higher the priority
+enum {
+  VALID        = 10,  ///< buffer not changed, or changes marked with b_mod_*
+  INVERTED     = 20,  ///< redisplay inverted part that changed
+  INVERTED_ALL = 25,  ///< redisplay whole inverted part
+  REDRAW_TOP   = 30,  ///< display first w_upd_rows screen lines
+  SOME_VALID   = 35,  ///< like NOT_VALID but may scroll
+  NOT_VALID    = 40,  ///< buffer needs complete redraw
+  CLEAR        = 50,  ///< screen messed up, clear it
+};
+
+/// While redrawing the screen this flag is set.  It means the screen size
+/// ('lines' and 'rows') must not be changed.
+EXTERN bool updating_screen INIT(= 0);
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "drawscreen.h.generated.h"
+#endif
+#endif  // NVIM_DRAWSCREEN_H