- Before slice delta QP was always 0. Now if global->QP is changed before
contexts are set, the delta qp is put to the bitstream, allowing for rough
frame level rate control.
- 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.