compress: improve rd 2 performance (Patch from Sumalatha)

a. Always allow intra mode for mode decision
b. increase more skips at each depth(do encode of best merge candidates initially and
if the merge is skipped, dont do inter modes)

Bitrate increase compared to rd 3 (4-6%) on most HD clips, quality maintained. Performance impact
to be measured in regression tests.

Sintel 720p clip needs to be investigated further. Poor show here.
Merge