use internalname for terrain layers

stops bugs like spaces in filenames and the like from occuring due to pointing right at the diffuse flat file name, as well as corruption from using a different diffuse in the material later

(cherry picked from commit c1227dbdd0dd61c25970eeaac128c6cbe5d25947)

Engine/source/terrain/terrFile.cpp

@@ -259,8 +259,7 @@ void TerrainFile::_initMaterialInstMapping()
    
    for( U32 i = 0; i < mMaterials.size(); ++ i )
    {
-      Torque::Path path( mMaterials[ i ]->getDiffuseMap());
-      mMaterialInstMapping.push_back( path.getFileName() );
+      mMaterialInstMapping.push_back(mMaterials[i]->getInternalName());
    }
    
    mMaterialInstMapping.mapMaterials();