transcode: set visible resolution correctly

Fix needed after b71c85b3d88b8
Encoders should use i_visible_width / i_visible_height

Refs #9147
Closed Captions: spawn a decoder thread

We can't keep the cc decoder serialized with video decoder.
If the decoding delay between packetizer and video decoder is bigger
than caching (for example when using multi threaded decoder), we will
be too late for the next picture display while we wait for the right
rendering time for the CC.

Close #9049