Make encoder emit EBU-TT-D docs with a specified timebase
Make encoder emit EBU-TT-D docs with a specified timebase

Fix #454 by emitting data with the `time_base` specified. Since we know
the document is an EBU-TT-D one it must be `media` which makes life
easier. An alternative might be to use whatever timebase is actually in
the document but that seems to be a pointless overhead.

bbc/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit,bbc/ebu-tt-live-toolkit,ebu/ebu-tt-live-toolkit