Improve fast mvd coding cost estimation
Improve fast mvd coding cost estimation

A lot of time is being taken up by this function on ultrafast, and it
doesn't do a very good job. This change aims to both simplify the
logic and make the estimate better.

The logic is simplified by using a look up for the step mvd bit cost
step function instead of mimicking the binarization process. The
estimation is made better by checking fractional cabac bit costs.

The new function returns the same results as
kvz_get_mvd_coding_cost_cabac, but is also faster than the old
function.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar