Use sqrt lambda cost for searches not using SSD.

- Add encoder_state->global->cur_lambda_cost_sqrt.

- Use sqrt lambda for inter search and rough intra search.

- The effect on inter is around 10-20% bdrate. The effect on intra is smaller
  and non-existent when --rd=2 is enabled, as the intra search refinement was
  already done with SSD and correct lambda.