Added lambda cost when using reference frames with refIdx > 0

Because selecting references "far away" costs bits, this should help with the issue that multiple reference frames actually increases coded bits with no quality gain.
Disabled NxN mode

Something must be done to enable better selection of NxN blocks in order to gain BD-rate with this mode.
Added number of reference frames to prefix SEI user data
Increased maximum ref frame count to 16 and set default to 3