Improve defaults for the GeoRef layer using the current view to suggest some values.
Fix tooltip of GeoRef northing / easting values.

It's the top left not the top right!
GeoRef Layer only works in UTM Projection Mode.

Only draw when in UTM Mode.
Add warning when adding an image and the current mode is not UTM mode as it will not be drawn.
Convert GeoRef failed image loading message into a GUI message.
[QA] These GeoRef layer functions should be static.
[QA] Tidy up DEM layer function definitions.

Remove from header as not used remotely.
Rename to remove vik_ prefix and make static.
Fix dem_layer_draw() definition to be consistent.
[QA] Consistent use of *_layer_draw parameter types.
Be less strigent in loading Geotagged images without optional GPS tags

Signed-off-by: Rob Norris <rw_norris@hotmail.com>