Fixes CNG to address the DTX issue reported by Gonzalo Mariano

- Bug fix: the CNG synthesis filter output consisted of only the prediction
  values, now added the input to that.
- CNG excitation level now takes into account the level from the PLC (since
  both PLC and CNG can be active at the same time).
- CNG is now called before PLC_glue() to smoothen the transition out of CNG.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>