rc: update vbv for all b frames

HEVC is complex (and slow) enough that we can afford to update/plan the VBV
buffer states for all frames, not just I and P. The leads to smoother rate
control, particularly when there are large stretches of B frames, and less
(over) compensation is necessary for P frames.

--HG--
extra : rebase_source : b0f673dae6f6d169b21007d351d1cca25cb3cb52
extra : amend_source : 56199889b38c6a678ba86a522e8045c6800c023c