refactor(options): use schar_T representation for fillchars and listchars

A bit big, but practically it was a lot simpler to change over all
fillchars and all listchars at once, to not need to maintain two
parallel implementations.

This is mostly an internal refactor, but it also removes an arbitrary
limitation: that 'fillchars' and 'listchars' values can only be
single-codepoint characters. Now any character which fits into a single
screen cell can be used.

src/nvim/grid.c

@@ -27,6 +27,7 @@
 #include "nvim/memory.h"
 #include "nvim/message.h"
 #include "nvim/option_vars.h"
+#include "nvim/optionstr.h"
 #include "nvim/types_defs.h"
 #include "nvim/ui.h"
 
@@ -67,7 +68,7 @@ void grid_adjust(ScreenGrid **grid, int *row_off, int *col_off)
   }
 }
 
-schar_T schar_from_str(char *str)
+schar_T schar_from_str(const char *str)
 {
   if (str == NULL) {
     return 0;
@@ -120,6 +121,13 @@ void schar_cache_clear(void)
 {
   decor_check_invalid_glyphs();
   set_clear(glyph, &glyph_cache);
+
+  // for char options we have stored the original strings. Regenerate
+  // the parsed schar_T values with the new clean cache.
+  // This must not return an error as cell widths have not changed.
+  if (check_chars_options()) {
+    abort();
+  }
 }
 
 bool schar_high(schar_T sc)
@@ -137,15 +145,39 @@ bool schar_high(schar_T sc)
 # define schar_idx(sc) (sc >> 8)
 #endif
 
-void schar_get(char *buf_out, schar_T sc)
+/// sets final NUL
+size_t schar_get(char *buf_out, schar_T sc)
+{
+  size_t len = schar_get_adv(&buf_out, sc);
+  *buf_out = NUL;
+  return len;
+}
+
+/// advance buf_out. do NOT set final NUL
+size_t schar_get_adv(char **buf_out, schar_T sc)
+{
+  size_t len;
+  if (schar_high(sc)) {
+    uint32_t idx = schar_idx(sc);
+    assert(idx < glyph_cache.h.n_keys);
+    len = strlen(&glyph_cache.keys[idx]);
+    memcpy(*buf_out, &glyph_cache.keys[idx], len);
+  } else {
+    len = strnlen((char *)&sc, 4);
+    memcpy(*buf_out, (char *)&sc, len);
+  }
+  *buf_out += len;
+  return len;
+}
+
+size_t schar_len(schar_T sc)
 {
   if (schar_high(sc)) {
     uint32_t idx = schar_idx(sc);
     assert(idx < glyph_cache.h.n_keys);
-    xstrlcpy(buf_out, &glyph_cache.keys[idx], 32);
+    return strlen(&glyph_cache.keys[idx]);
   } else {
-    memcpy(buf_out, (char *)&sc, 4);
-    buf_out[4] = NUL;
+    return strnlen((char *)&sc, 4);
   }
 }
 
@@ -433,14 +465,13 @@ int grid_line_puts(int col, const char *text, int textlen, int attr)
   return col - start_col;
 }
 
-void grid_line_fill(int start_col, int end_col, int c, int attr)
+void grid_line_fill(int start_col, int end_col, schar_T sc, int attr)
 {
   end_col = MIN(end_col, grid_line_maxcol);
   if (start_col >= end_col) {
     return;
   }
 
-  schar_T sc = schar_from_char(c);
   for (int col = start_col; col < end_col; col++) {
     linebuf_char[col] = sc;
     linebuf_attr[col] = attr;
@@ -532,11 +563,17 @@ void grid_line_flush_if_valid_row(void)
   grid_line_flush();
 }
 
+void grid_clear(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, int attr)
+{
+  grid_fill(grid, start_row, end_row, start_col, end_col, schar_from_ascii(' '),
+            schar_from_ascii(' '), attr);
+}
+
 /// Fill the grid from "start_row" to "end_row" (exclusive), from "start_col"
 /// to "end_col" (exclusive) with character "c1" in first column followed by
 /// "c2" in the other columns.  Use attributes "attr".
-void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, int c1,
-               int c2, int attr)
+void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int end_col, schar_T c1,
+               schar_T c2, int attr)
 {
   int row_off = 0;
   int col_off = 0;
@@ -582,7 +619,7 @@ void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int
     }
 
     int col = start_col;
-    schar_T sc = schar_from_char(c1);
+    schar_T sc = c1;
     for (col = start_col; col < end_col; col++) {
       size_t off = lineoff + (size_t)col;
       if (grid->chars[off] != sc || grid->attrs[off] != attr || rdb_flags & RDB_NODELTA) {
@@ -595,7 +632,7 @@ void grid_fill(ScreenGrid *grid, int start_row, int end_row, int start_col, int
       }
       grid->vcols[off] = -1;
       if (col == start_col) {
-        sc = schar_from_char(c2);
+        sc = c2;
       }
     }
     if (dirty_last > dirty_first) {
@@ -683,7 +720,7 @@ void grid_put_linebuf(ScreenGrid *grid, int row, int coloff, int col, int endcol
         col++;
       }
       if (col <= endcol) {
-        grid_fill(grid, row, row + 1, col + coloff, endcol + coloff + 1, ' ', ' ', bg_attr);
+        grid_clear(grid, row, row + 1, col + coloff, endcol + coloff + 1, bg_attr);
       }
     }
     col = endcol + 1;