wl: counting bytes needs no buffer to put them in

backbytes copied the runes into a Str and encoded them into a 257-byte
stack buffer only to throw the bytes away and keep the length.  runelen
answers over the same runes with neither.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

wl.c

@@ -127,14 +127,12 @@ sendrequest(int op, u32int ks, u32int mod, Keyres *res)
 static int
 backbytes(int n)
 {
-	char utf[Maxutf];
-	Str s;
-	int i;
+	int i, nb;
 
-	sclear(&s);
+	nb = 0;
 	for(i = max(surround.n - n, 0); i < surround.n; i++)
-		sputr(&s, surround.r[i]);
-	return stoutf(&s, utf, sizeof utf);
+		nb += runelen(surround.r[i]);
+	return nb;
 }
 
 /* The engine's modifier bits, read from the grab's own keyboard state. */