- 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.
- It's good that this module has been chopped to smaller pieces, but lets
avoid including .c files unless we really have to. These make pretty good
submodules on their own so just make them their own compilation units.
- Move some stuff around to avoid having to forward declare them
in encoderstate.c.