srtenc: Add timing-less "subrip" encoder.

Unsurprisingly, if a timing-less subrip decoder is desireable, an
encoder is as well. With this in place, we can move on to remove
the use of the old encoder/decoder with embedded timing and move
all timing handling the (de)muxer where they belong.

Signed-off-by: Philip Langdale <philipl@overt.org>

tests/ref/fate/sub-subripenc

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