Further adjustment of RD behaviour with Q and Zbin.
Following conversations with Tim T (Derf) I ran a large number of tests comparing the existing polynomial expression with a simpler ^2 variant. Though the polynomial was sometimes a little better at the extremes of Q it was possible to get close for most clips and even a little better on some. This code also changes the way the RD multiplier is calculated when the ZBIN is extended to use a variant of the same ^2 expression. I hope that this simpler expression will be easier to tune further as we expand our test set and consider adjustments based on content. Change-Id: I73b2564346e74d1332c33e2c1964ae093437456c