inputpanel: ignore requested output

While there's specific hardware where the IM could benefit from requesting to
be shown on a specific output, it effectively never has enough information to
choose a useful output - and the protocol doesn't allow setting a null output
to indicate that the compositor should do the choice.
To avoid showing the OSK on the wrong output, always put it on the active
output and ignore what the IM client requests.


(cherry picked from commit 7cb4b51d)