transcoding video, sends NULL when ending. Should fix sending all the encoded frames to sout

Sends NULL-pict to encode when it's closing time, then encoder knows to
flush buffers and check that it has outputted all the frames.

Contains changes to x264/avcodec-module to implement that on encoder-side.
Add quick check on omxil/dirac/theora for that, so they don't crash.
And add likely-macro to x264/avcodec in check of NULL

If someone more familiar with dirac/theora/omxil encoder-modules could
check if they have buffers that need to be outputted, would be nice.

Fixes some tickets, but I failed to find any of those in trac.