Revert "pthread: flush all threads on flush, not just the first one"

This reverts commit 2eb15cdeef29eb8a0a32658154decba94b4b89cb.

It does not work correctly in pre-refcounting threading code.
h264: reset next_output_pic earlier in start_frame()

In case start_frame() fails, this potentially invalid frame can still be
output to the caller.

Bug-Id: 672
Bug-Id: debian/741240
Bug-Id: ubuntu/1288206