ffmpeg: fix -stream_loop with multiple inputs

The input thread needs to be properly cleaned up and re-initalized before we
can start reading again in threaded mode. (Threaded input reading is used when
there is mode than one input file).

Fixes ticket #6121 and #7043.

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit da36bcbe)