Add signaling for a maximum DRED quantizer.

Since any value of dQ > 0 will cause the initial quantizer to degrade to the format-implied maximum (15) with a sufficient number of DRED frames, allow signaling a maximum smaller than 15. This allows encoders to improve the minimum quality of long DRED sequences (at the expense of bitrate) without requiring a constant quantizer for all frames (dQ == 0).