Enable topskip and earlyexit for all rd levels <= 4 (output changes for presets faster than "slow")

Also use the encodeResandCalcRDInter instead of the refactored estimate function.
rdlevel: Add code for rdlevel 2

Use signalling bits + sa8d cost to choose best among inter/merge/intra. Encode only best mode at each depth.
rdlevel: compare Merge-skip(merge2Nx2N with no residue) to best among inter/intra/merge in rdlevel 2
rdlevel: skip Intra if inter/merge sa8d less than a threshold

In higher rdlevels Intra is skipped if inter/merge cu cbf is 0. A threshold of sa8d expects that cu cbf will be 0.
Thresholds have to be refined further.