RD: change cost measurements for inter

The cost calculation for inter modes now use cost = satd(orig, pred) + lambda*mebits.
This is an estimation to the actual RD-cost.
RD: use sa8d in inter/merge cost measurements instead of satd
merge2nx2n: move initializations out of the loop

Only mergeIndex and MVs will be changed in each loop.
RD merge: remove more redundant set-fields