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).