WebAdmin: Fix off-by-one area errors about gallery HOT 1 OPEN

The root of the problem is that the plugin is trying to export too large an area as the image. Instead of exporting only the bounding-box of the changed voxels, it exports the entire gallery area, including the ~180 air blocks above any build and all the way down to the bedrock layer.
The original idea for the export coords is as follows:

1. If the area has a user-defined bounding box (as maintained in the shared DB by the GalExport plugin), use that.
2. If not, use the stored min and max coords of the edits (each block placement and removal should update these).
3. If not (for areas created before the edit-tracking code), calculate the diff of the area with the original area data (as set in the gallery's template schematic)
   It seems part of this decision code is missing, and the third step has the off-by-one problem when diffing.

from gallery.