Slight tuning of 96 kbps stereo IGF config

The IGF config tuned for 96 kbps stereo, but also used in e.g. 128 kbps SBA, currently uses a very high source region (ca. 9-12 kHz) for copy-up. For some special cases where the source region is almost empty but the target region (12.8-16 kHz) has much more energy again, this can lead to problems, like one in a Dolby test item where noticeable pre-echoes occurred on some transients due to this issue.

It is therefore the safer choice to lower the source region by about 3 kHz. That solves the mentioned issue with the Dolby item and should make these sorts of issues less likely to occur, in general.

This change will affect all modes using this IGF config, but should have only minimal perceptual impact on the vast majority of items.