engine: a takeover keeps the text it took for the context it took it from

Two applications' focus events cross — ibus-daemon documents the case —
so the first key of the new one can arrive before the old one says it
lost focus.  The engine dropped whatever that context was composing; it
now keeps it and hands it back with the reset or release that follows,
which every frontend already commits.  Normal ordering is unchanged.

tests/server_test.c

@@ -284,7 +284,8 @@ server_connection_ownership(struct ct *t)
 	req = nextrequest(t, &f.pump, Keyreset);
 	CT_EQ_PTR(t, aowner, req.owner);
 	allowrequest(&f.pump);
-	CT_CHECK(t, readreply(t, &a, 1, "", preedit));
+	/* The reset hands back what the takeover took from this client. */
+	CT_CHECK(t, readreply(t, &a, 1, "か", preedit));
 	CT_EQ_STR(t, "", preedit);
 	CT_EQ_PTR(t, bowner, testengineowner());
 	if(!sendkey(t, &b, 1, 0, 0))