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/engine_test.c

@@ -234,6 +234,7 @@ engine_active_owner_lifecycle(struct ct *t)
 	checkstr(t, "takeover resets old preedit", "ん", &res.preedit);
 	CT_EQ_PTR(t, &b, activeowner);
 	res = ownerrequest(&a, Keyreset, 0, 0);
+	checkstr(t, "the taken owner gets its reading back", "か", &res.commit);
 	CT_EQ_INT(t, 0, res.preedit.n);
 	impre(&im, &shown);
 	checkstr(t, "stale reset preserves owner", "ん", &shown);