lavc/ccaption_dec: implement real_time option

This new mode is useful for realtime decoding of closed captions so they
can be display along with mpeg2 frames.

Closed caption streams contain two major types of captions:

- POPON captions, which are buffered off-screen and displayed
  only after EOC (end of caption, aka display buffer)

- PAINTON/ROLLUP captions, which are written to the display as soon as
  they arrive.

In a typical real-time eia608 decoder, commands like EOC (end of
caption; display buffer), EDM (erase display memory) and EBM (erase
buffered memory) perform their expected functions as soon as the
commands are processed. This is implemented in the real_time branches
added in this commit.

Before this commit, and in the !real_time branches after this commit,
the decoder cleverly implements its own version of the decoder which is
specifically geared towards buffered decoding. It does so by actively
ignoring commands like EBM (erase buffered memory), and then re-using
the non-display buffer to hold the previous caption while the new one is
received. This is the opposite of the real-time decoder, which uses the
non-display buffer to hold the new caption while the display buffer is
still showing the current caption.

In addition to ignoring EBM, the buffered decoder also has custom
implementations for EDM and EOC. An EDM (erase display memory) command
flushes the existing contents before clearing the screen, and EOC
similarly always flushes the active buffer (the previous subtitle)
before flipping buffers.

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