Fix lossless transcode.

Following 6c3653cd378aacef10b7706d8b7f71beae41274c, lossless mythtranscode got broken.

This is just a quick workaround, the whole code needs to be revisited and properly use FFmpeg API

Fixes #11213

Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>
(cherry picked from commit 2a27e176275155a04498ecb8eac67dbd0164d080)