Incorrect bit allocation in forced KF groups.

Incorrect bit allocation in forced KF groups.

The old 2 pass code estimated error distribution when coding a
forced (by interval) key frame. The result of this was that in some
cases, when allocating bits at the GF group level within a KF
group there was either a glut of bits or starvation of bits at the end
of the KF group.

Added code to rescan and get the correct data once the position of
a forced key frame has been determined.

Change-Id: I0c811675ef3f9e4109d14bd049d7641682ffcf11