MetaStream::Video dropped color_space on from_title/to_title, hardcoding
BT.709 on the way back. HDR titles (BT.2020) lost their color metadata.
Add a color_space field, populate it in from_title, and use it in
to_title. For pre-0.30.7 metadata that has no color_space, derive it from
the preserved hdr field (all HDR formats are BT.2020, SDR is BT.709).
Adds ColorSpace::id() + FromStr for serialization round-trip.
MattJackson
97ae452e40