Fix cursor move with wide glyphs

st would always move back 1 column,
even with wide glyhps (using more than a single column).

The glyph rune is set on its first column,
and the other ones are to 0,
so loop until we detect the start of the previous glyph.

ref.
https://git.suckless.org/st/commit/7473a8d1a57e5f9aba41b953f4e498c35e1c9dc5.html

st.c

@@ -3079,12 +3079,16 @@ tstrsequence(uchar c)
 void
 tcontrolcode(uchar ascii)
 {
+	size_t i;
+
 	switch (ascii) {
 	case '\t':   /* HT */
 		tputtab(1);
 		return;
 	case '\b':   /* BS */
-		tmoveto(term.c.x-1, term.c.y);
+		for (i = 1; term.c.x && term.line[term.c.y][term.c.x - i].u == 0; ++i)
+			;
+		tmoveto(term.c.x - i, term.c.y);
 		return;
 	case '\r':   /* CR */
 		tmoveto(0, term.c.y);

st.h

@@ -155,8 +155,8 @@ typedef struct {
 
 typedef struct {
 	Glyph attr; /* current char attributes */
-	int x;
+	int x; /* terminal column */
-	int y;
+	int y; /* terminal row */
 	char state;
 } TCursor;