optimize column width calculation and utf-8 encode for ASCII

In particular on OpenBSD and on glibc wcwidth() is quite expensive.
On musl there is little difference.

st.c

@@ -2307,7 +2307,7 @@ tputc(Rune u)
 	Glyph *gp;
 
 	control = ISCONTROL(u);
-	if (!IS_SET(MODE_UTF8 | MODE_SIXEL)) {
+	if (u < 127 || !IS_SET(MODE_UTF8 | MODE_SIXEL)) {
 		c[0] = u;
 		width = len = 1;
 	} else {