Independent advanced textures

[daves-ssu]

Implement independent advanced textures that can be used separately from the main diffuse channel texture of the mesh(es). This would enable use of common advanced textures for several separate diffuse channel textures (see SSU/SSV orbiter texture loading, one common normal texture for Columbia/Challenger which are similar enough, same for Discovery, Atlantis and Endeavour).

[jarmonik]

SURFHANDLE oapiLoadAdditionalTextureMaps(const char* diff, const char* maps = NULL, bool bPath = false, SURFHANDLE hOld = NULL, bool bAll = true);

Function has been added to API. Not yet tested. Need to test that everything works fine.
TODO: Need to add a load of regular diffuse texture in D3D7