Adds OPUS_SET_PREDICTION_DISABLED() ctl to force "independent" frames

Works by turning off pitch and energy prediction in CELT, while
setting first_frame_after_reset in SILK to disable pitch and LSF interpolation
and reduce LPC gain.
diff --git a/src/opus_multistream_encoder.c b/src/opus_multistream_encoder.c
index 7cb1af7..cec6266 100644
--- a/src/opus_multistream_encoder.c
+++ b/src/opus_multistream_encoder.c
@@ -1028,6 +1028,7 @@
    case OPUS_GET_SAMPLE_RATE_REQUEST:
    case OPUS_GET_INBAND_FEC_REQUEST:
    case OPUS_GET_FORCE_CHANNELS_REQUEST:
+   case OPUS_GET_PREDICTION_DISABLED_REQUEST:
    {
       OpusEncoder *enc;
       /* For int32* GET params, just query the first stream */
@@ -1073,6 +1074,7 @@
    case OPUS_SET_DTX_REQUEST:
    case OPUS_SET_FORCE_MODE_REQUEST:
    case OPUS_SET_FORCE_CHANNELS_REQUEST:
+   case OPUS_SET_PREDICTION_DISABLED_REQUEST:
    {
       int s;
       /* This works for int32 params */