[Bug] "ungeoreferenced" region retains GCPs after being re-split

[mradamcox]

To Reproduce
Steps to reproduce the behavior:

1. Georeference a Region that has not been split
2. Ungeoreference it
3. Unprepare it
4. Split the Region into two or more pieces
5. Georeference one of the pieces (possibly it must be the [1] piece)
6. You'll see that the GCPs appear in the georeferencing interface, but because the dimension of the document have changed, they show an offset/incorrect preview

Expected behavior

Steps 1-5 above should result in a clear georeferencing interface.

Additional context

The first place to look will be how the points are acquired and furnished to the georeferencing interface (can't remember ATM if it's from GCPGroup or GeorefSession.data).