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/drawline.h

@@ -0,0 +1,24 @@
+#ifndef NVIM_DRAWLINE_H
+#define NVIM_DRAWLINE_H
+
+#include "nvim/decoration_provider.h"
+#include "nvim/fold.h"
+#include "nvim/screen.h"
+
+// Maximum columns for terminal highlight attributes
+#define TERM_ATTRS_MAX 1024
+
+typedef struct {
+  NS ns_id;
+  uint64_t mark_id;
+  int win_row;
+  int win_col;
+} WinExtmark;
+EXTERN kvec_t(WinExtmark) win_extmark_arr INIT(= KV_INITIAL_VALUE);
+
+EXTERN bool conceal_cursor_used INIT(= false);
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "drawline.h.generated.h"
+#endif
+#endif  // NVIM_DRAWLINE_H