Support passing through escape sequences to the underlying terminal by

using DCS with a "tmux;" prefix. Escape characters in the sequences must
be doubled. For example:

$ printf '\033Ptmux;\033\033]12;red\007\033\\'

Will pass \033]12;red\007 to the terminal (and change the cursor colour
in xterm). From Kevin Goodsell.

screen-write.c

@@ -1191,3 +1191,15 @@ screen_write_overwrite(struct screen_write_ctx *ctx, u_int width)
 		grid_view_set_cell(gd, xx, s->cy, &grid_default_cell);
 	}
 }
+
+void
+screen_write_rawstring(struct screen_write_ctx *ctx, u_char *str, u_int len)
+{
+	struct tty_ctx		 ttyctx;
+
+	screen_write_initctx(ctx, &ttyctx, 0);
+	ttyctx.ptr = str;
+	ttyctx.num = len;
+
+	tty_write(tty_cmd_rawstring, &ttyctx);
+}