Nominated for the price of the ugliest commitdiff

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

It only works for blend and X mode but there are probably the most used.
When blend/X are enabled/disabled:
 - It avoids recreating the vout.
 - It avoids deinterlacing the (soft) subtitles.
The other modes are not possible without a lot of vout works, so it is
postponed for later.

As a side effect, the initial value of the menu now takes into account the
command line options.

Please, report any regression. This code is made complicated by the
vout filter/video filter2 switches&tricks.

The current code assume it everywhere.

It now supports I422 and discard mode.

It is only used from a single thread

And hmm, this really should use a condition variable too.