When a decoded stream is being looped, after each post-EOF rewind,
decoders are flushed in seek_to_start(). This only drains 1 frame, and
thus the output has a few frames missing at the tail of each iteration
except the last.

With this patch, process_input is looped till process_input_packet
reaches EOF.

Fixes #7081
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>