Modified so that a multiple-element FG_SCENERY path will work:

Modified so that a multiple-element FG_SCENERY path will work:

1. Do not stop scanning STG files after OBJECT_BASE is found.

2. Load OBJECT_BASE only once.

3. Load OBJECT only when no OBJECT_BASE has been found or when
   OBJECT_BASE was found in the same file (probably should be only the
   latter, if we constrain OBJECT_BASE always to come first).

4. Always load OBJECT_STATIC and OBJECT_SHARED.