Use the new `OBS_ENCODER_CAP_MULTITRACK_DYN_BITRATE` flag when
streaming with multiple tracks, instead of relying on the single-
track `OBS_ENCODER_CAP_DYN_BITRATE` flag. In multitrack cases where
multiple codecs are being used (e.g. mixed AVC and HEVC), all encoders
must support the multitrack capability flag otherwise the dynamic
bitrate (DBR) feature is disabled for the streaming session.
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