refactor(grid): move out grid_* functions from screen.c

Also normalize some types. use "size_t" for unsigned array offsets.
Fix -Wconversion issues missed as screen.c is missing this check.

src/nvim/grid.h

@@ -0,0 +1,57 @@
+#ifndef NVIM_GRID_H
+#define NVIM_GRID_H
+
+#include <stdbool.h>
+
+#include "nvim/ascii.h"
+#include "nvim/grid_defs.h"
+#include "nvim/buffer_defs.h"
+
+/// By default, all windows are drawn on a single rectangular grid, represented by
+/// this ScreenGrid instance. In multigrid mode each window will have its own
+/// grid, then this is only used for global screen elements that hasn't been
+/// externalized.
+///
+/// Note: before the screen is initialized and when out of memory these can be
+/// NULL.
+EXTERN ScreenGrid default_grid INIT(= SCREEN_GRID_INIT);
+
+#define DEFAULT_GRID_HANDLE 1  // handle for the default_grid
+
+EXTERN schar_T *linebuf_char INIT(= NULL);
+EXTERN sattr_T *linebuf_attr INIT(= NULL);
+
+// Low-level functions to manipulate individual character cells on the
+// screen grid.
+
+/// Put a ASCII character in a screen cell.
+static inline void schar_from_ascii(char_u *p, const char c)
+{
+  p[0] = (char_u)c;
+  p[1] = 0;
+}
+
+/// Put a unicode character in a screen cell.
+static inline int schar_from_char(char_u *p, int c)
+{
+  int len = utf_char2bytes(c, p);
+  p[len] = NUL;
+  return len;
+}
+
+/// compare the contents of two screen cells.
+static inline int schar_cmp(char_u *sc1, char_u *sc2)
+{
+  return strncmp((char *)sc1, (char *)sc2, sizeof(schar_T));
+}
+
+/// copy the contents of screen cell `sc2` into cell `sc1`
+static inline void schar_copy(char_u *sc1, char_u *sc2)
+{
+  xstrlcpy((char *)sc1, (char *)sc2, sizeof(schar_T));
+}
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "grid.h.generated.h"
+#endif
+#endif  // NVIM_GRID_H