libutvideoenc: Add support for the new BT.709 FourCCs for YCbCr

With cli usage the decoder might have not set the colorspace during
encoder init, manual colorspace override might be needed in such
cases.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

This applies commit 5de64bb3 (the source of the above commit message)
to libutvideoenc as well.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>