scenery: rework stg loading code.

scenery: rework stg loading code.

This change reworks the code block loading the stg files.
It fixes a long standing problem where we end up loading
similar models from different entries of the scenery
path at the same geographic location. This usually results
in models that appear to have z figting, even if the isolated
model does not show z fighting at all.
As a result the lookup sequence for models and scenery has
changed a bit. This should be closer to what it supposed to
be and did not show any problems on the areas and sceneries
I tested.