1 pass VBR mode bug fix.
(copied from VP9) The one pass VBR mode selects a Q range based on a moving average of recent Q values. This calculation should have been excluding arf overlay frames as these are usually coded at the highest allowed value. Their inclusion skews the average and can cause it to drift upwards even when the clip as a whole is undershooting. As such it can undermine correct adaptation of the allowed Q range especially for easy content. Change-Id: I9e12da84e12917e836b6e53ca4dfe4f150b9efb1