Store the total budget of compute_allocation in BITRES units.

The margin of safety was supposed to be 1/8th bit, not 1 bit, and the
 bit we reserved to terminate skip signalling before was actually 8
 bits.
This patch updates the margin of safety to the correct value and
 accounts for the one bit (not 8) needed for skip signalling.
It also fixes the remainder calculation in the skip loop to work
 correctly when start>0.