[Task] Ensure mosaic process uses GCPs from database, not session.data

[mradamcox]

Describe the task Currently, the mosaic process creates a VRT for each layer using the cutline from the multimask, before turning all of those VRTs into a single mosaic file. This process should use the GCPs that are stored in the database for the document, not those that are stored in the latest georeferencing session's data.

See https://github.com/ohmg-dev/OldInsuranceMaps/issues/184 for more context