compress: Fixes fudging in merge costs, stores sa8d costs for bestMergeCU in m_sa8dCost

--HG--
extra : amend_source : 0f6c313b1cf0ab18ea19e4c7cec0666a29f17aae
compress: save inter sa8d costs also in m_sa8dCost
compress: save intra costs in rd <= 2 also in m_sa8dCost.

Basically, all sa8d costs are always saved in m_sa8dCost.
rest: document lossless coding theory and behavior
encoder: insert missing comma