If transcoding, don't discard frames during a seek operation

This is necessary because trancoding uses a queue of frames, and frames
held in the queue can be reused and overwritten if DiscardFrames is
called.

Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>
Observed behavior: exit playback, black screen you could not interact with, only recourse was to restart mythfrontend.

Fix the mutex lock count. It would get off by one for every EAGAIN preventing the player teardown form completing (lock in AvFormatDecoder::CloseCodecs that could never be obtained).

Signed-off-by: Jean-Yves Avenard <jyavenard@mythtv.org>