[non-BE]MDCT-Stereo PLC fadeout improvements contribution
See https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/wikis/Contributions/1-mdct-stereo-plc-fadeout for a description of the contribution. Since the code was already part of the initial baseline, this MR includes only some recent changes that fix problems with MCT modes and would be mainly used to eventually activate the define (MDCT_STEREO_PLC_FADE_2_BG_NOISE).
Edited by Jan Kiene