The limits applied under the flag
"LIMIT_QRANGE_FOR_ALTREF_AND_KEY"
behaved in an undesirable way if the gap between
active_worst_quality and active_best_quality was
changed.

In this patch, the adjustment is made using the
vp9_compute_qdelta_by_rate() function and fixed
rate multiplier values. Hence it is not impacted by
the relative value of active_best_quality.

Change-Id: I93b3308e04ade1e4eb5af63edf64f91cd3700249