In the old code, the function of allocate_gf_group_bits() in
firstpass.c handles both GF group structure design as well as the
corresponding bit allocation for each coded frame. This CL separates
the two functionalities.

It has been verified that no coding performance has been incurred by
this CL.

Change-Id: I544b4fcdc8cb37ec810373f1f820dbd2e400c39b