VTK: Invert IJK indices to match ResInsight convention

VTK coordinates are negated (z positive downward) to match ResInsight
internal convention, which flips the IJK directions relative to what the
structured grid algorithm expects. Add invertIJK flag to RigFemPartGrid
and RigFemPart so the inversion is applied during struct grid construction.

ApplicationLibCode/GeoMech/GeoMechFileInterface/RifVtkReader.cpp

@@ -191,6 +191,10 @@ bool RifVtkReader::readFemParts( RigFemPartCollection* femParts )
             femPart->addElementSet( setName, elementSet );
         }
 
+        // VTK coordinates are negated to match ResInsight internal convention (z positive downward),
+        // which flips the IJK directions. Invert all so IJK=0 is at the correct corner.
+        femPart->setInvertIJKInStructGrid( true );
+
         femPart->setElementPartId( femParts->partCount() );
         femParts->addFemPart( femPart );