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

terminal.h

@@ -87,6 +87,7 @@ struct damage {
 struct composed {
     wchar_t chars[20];
     uint8_t count;
+    int width;
 };
 
 struct row_uri_range {