MDCT Stereo Concealment: high-band artefacts after DTX

There is currently a problem in MDCT stereo when the first active frame after CNG is lost, and the subsequent MDCT Stereo frames is forced to be a transition-frame. The noise-shaping might miss out on the IGF data.

... | inactive | inactive | MDCT Stereo | MDCT Stereo | ...
                                 ^
                                lost

The problem is not so severe in float, but appears rather drastic in FX, since there also the scaling is adapted, see also sa4/audio/ivas-basop#1997 (closed).