refactor(grid): make screen rendering more multibyte than ever before

Problem: buffer text with composing chars are converted from UTF-8
to an array of up to seven UTF-32 values and then converted back
to UTF-8 strings.

Solution: Convert buffer text directly to UTF-8 based schar_T values.

The limit of the text size is now in schar_T bytes, which is currently
31+1 but easily could be raised as it no longer multiplies the size
of the entire screen grid when not used, the full size is only required
for temporary scratch buffers.

Also does some general cleanup to win_line text handling, which was
unnecessarily complicated due to multibyte rendering being an "opt-in"
feature long ago. Nowadays, a char is just a char, regardless if it consists
of one ASCII byte or multiple bytes.

src/nvim/eval.c

@@ -7117,7 +7117,7 @@ dict_T *get_vim_var_dict(int idx) FUNC_ATTR_PURE
 /// Set v:char to character "c".
 void set_vim_var_char(int c)
 {
-  char buf[MB_MAXBYTES + 1];
+  char buf[MB_MAXCHAR + 1];
 
   buf[utf_char2bytes(c, buf)] = NUL;
   set_vim_var_string(VV_CHAR, buf, -1);