This is mainly just for testing until we get the database and proper cutter up,
but it might prove useful to have in the long run too.
This code will probably end up being totally rewritten,
as it uses the most naive form of cutting and reencoding,
and it has a whole bunch of http-serving specifics intertwined with the cutting logic.
This prevents clients from picking a variant that they then can't play any content for.
In general we expect the same content to be available on all variants being captured,
but if the set of captured variants changes we still want to handle that gracefully.