decoder: factorize the visible size fixup during decoder format update

This way other decoder owner (thumbnail, transcode, etc) benefit from the same
fix in a consistent manner.

Now we clean the value directly in the dec->fmt_out whereas before it was
cleaned at the vout level and the decoder didn't know about it.

In the end if the decoder didn't write it before it would not have written it
later anyway. And if it really wants to, it can still do it later and issue an
update_format.