Bandwidth in 1ISM at 16.4 kbps

This is a follow-up of #556 (closed).

The audio bandwidth in IVAS ISM coding is lower than that in EVS at low bitrates. E.g. for 3ISM there are:

bitrate	EVS	IVAS
48 kbps	FB	SWB (cut-off 14.4 kHz)
64 kbps	FB	SWB (cut-off 16 kHz)

When changing the source code and enabling FB coding from 16 kbps per object, i.e. changing macro MIN_BRATE_FB_ISM from 24000 to 16000, the bandwidth of IVAS would match that of EVS. The change of the mentioned macro would affect these operating points:

• 1ISM at 16.4 kbps
• 2ISM at 32 kbps
• 3ISM at 48 kbps and 64 kbps
• 4ISM at 64 kbps and 80 kbps

It is a question whether IVAS should match the EVS audio bandwidth at low bitrates or if it is preferred to keep the status quo.

An experimental change is available in MR !788 (merged) under ISM_FB_16k4.