fcft: adapt to API changes in fcft-3.x

Fcft no longer uses wchar_t, but plain uint32_t to represent
codepoints.

Since we do a fair amount of string operations in foot, it still makes
sense to use something that actually _is_ a string (or character),
rather than an array of uint32_t.

For this reason, we switch out all wchar_t usage in foot to
char32_t. We also verify, at compile-time, that char32_t used
UTF-32 (which is what fcft expects).

Unfortunately, there are no string functions for char32_t. To avoid
having to re-implement all wcs*() functions, we add a small wrapper
layer of c32*() functions.

These wrapper functions take char32_t arguments, but then simply call
the corresponding wcs*() function.

For this to work, wcs*() must _also_ be UTF-32 compatible. We can
check for the presence of the  __STDC_ISO_10646__ macro. If set,
wchar_t is at least 4 bytes and its internal representation is UTF-32.

FreeBSD does *not* define this macro, because its internal wchar_t
representation depends on the current locale. It _does_ use UTF-32
_if_ the current locale is UTF-8.

Since foot enforces UTF-8, we simply need to check if __FreeBSD__ is
defined.

Other fcft API changes:

* fcft_glyph_rasterize() -> fcft_codepoint_rasterize()
* font.space_advance has been removed
* ‘tags’ have been removed from fcft_grapheme_rasterize()
* ‘fcft_log_init()’ removed
* ‘fcft_init()’ and ‘fcft_fini()’ must be explicitly called

xmalloc.c

@@ -3,7 +3,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <wchar.h>
 #include "xmalloc.h"
 #include "debug.h"
 
@@ -45,18 +44,18 @@ xstrdup(const char *str)
     return check_alloc(strdup(str));
 }
 
-wchar_t *
-xwcsdup(const wchar_t *str)
-{
-    return check_alloc(wcsdup(str));
-}
-
 char *
 xstrndup(const char *str, size_t n)
 {
     return check_alloc(strndup(str, n));
 }
 
+char32_t *
+xc32dup(const char32_t *str)
+{
+    return check_alloc(c32dup(str));
+}
+
 static VPRINTF(2) int
 xvasprintf_(char **strp, const char *format, va_list ap)
 {