Correct shift amount on MODE_INSERT in tputc()

When MODE_INSERT is set we'd shift characters on the same
line forward before inserting our character in tputc().
This did not account for wide characters where width != 1.
This patch makes it so we shift the correct amount.

st.c

@@ -2676,8 +2676,8 @@ tputc(char *c, int len) {
 		gp = &term.line[term.c.y][term.c.x];
 	}
 
-	if(IS_SET(MODE_INSERT) && term.c.x+1 < term.col)
-		memmove(gp+1, gp, (term.col - term.c.x - 1) * sizeof(Glyph));
+	if(IS_SET(MODE_INSERT) && term.c.x+width < term.col)
+		memmove(gp+width, gp, (term.col - term.c.x - width) * sizeof(Glyph));
 
 	if(term.c.x+width > term.col) {
 		tnewline(1);