vt: cache grapheme cluster width in composed struct

* Use regular wcswidth() to calculate the width
* Explicitly set to ‘2’ if we see a emoji variant selector
* Cache the result in the composed struct

util.h

@@ -35,11 +35,3 @@ sdbm_hash(const char *s)
 
     return hash;
 }
-
-#include <wchar.h>
-static inline int
-my_wcswidth(const wchar_t *s, size_t n)
-{
-    int ret = wcswidth(s, n);
-    return max(0, ret);
-}