Re-instated the Geoscience Aus layer into default_bathy

Fixes the portal issue #1338 where the basemap default bathy is blurry.
The bathy tiff is switched off now when zoomed right in, so only the underlying layer (flat blue) is shown
The missing tiff layer around australia is re-instated. This is switched on when zooming in and also is switch off when zoomed right in.