Fix extra bracketed paste markers when pasting >8kb

Before this patch, when pasting over BUFSIZE (8192 bytes here), st would
do the following:

    \e[200~...8192 bytes...\e[201~\e[200~...remaining bytes...\e[201~

With this patch, the start marker is only sent when the offset is 0 (at
the beginning of selnotify) and the end marker is only sent when the
remaining bytes to read are 0 (at the end).

For short pastes, both conditions are true in the same iteration.

For long pastes, it removes the extra markers in the middle, keeping the
intended wrapping:

    \e[200~...8192 bytes......remaining bytes...\e[201~

Signed-off-by: Christoph Lohmann <20h@r-36.net>

st.c

@@ -1135,10 +1135,10 @@ selnotify(XEvent *e)
 			*repl++ = '\r';
 		}
 
-		if (IS_SET(MODE_BRCKTPASTE))
+		if (IS_SET(MODE_BRCKTPASTE) && ofs == 0)
 			ttywrite("\033[200~", 6);
 		ttysend((char *)data, nitems * format / 8);
-		if (IS_SET(MODE_BRCKTPASTE))
+		if (IS_SET(MODE_BRCKTPASTE) && rem == 0)
 			ttywrite("\033[201~", 6);
 		XFree(data);
 		/* number of 32-bit chunks returned */