Bug in clamping of base_frame_target.

Bug relating to issue:- http://b/25090786

base_frame_target is supposed to track the idealized bit
allocation based on error score and not the actual bits
allocated to each frame.

The clamping of this value based on the VBR min and max pct values
was causing a bug where in some cases the loop that adjusts the
active max quantizer for each GF group was running out of bits at
the end of a KF group. This caused a spike in Q and some ugly artifacts.

A second change makes sure that the calculation of the active
Q range for a group DOES, however, take account of clamping.

Change-Id: I31035e97d18853530b0874b433c1da7703f607d1