Fill a new SessionProperties member "forcedVideoCodecResolved" to
contain the resolved value of "forcedVideoCodec", once the new
MEDIA_SERVER_PREFERRED has been taken into account and translated into
the appropriate option for each media server.
The logic to decide how to translate MEDIA_SERVER_PREFERRED into a
concrete forcedVideoCodec value is placed once in the REST entry point
of session creation (SessionRestController.java). Afterwards,
SessionProperties is just used as a simple storage for all session
features, and serialized / passed around between server and client.
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