fix(rt): dont convert to volume for RTSTRUCT (#4157)

Co-authored-by: Alireza <ar.sedghi@gmail.com>

extensions/cornerstone-dicom-rt/src/viewports/OHIFCornerstoneRTViewport.tsx

@@ -111,7 +111,7 @@ function OHIFCornerstoneRTViewport(props: withAppTypes) {
         {...props}
         displaySets={[referencedDisplaySet, rtDisplaySet]}
         viewportOptions={{
-          viewportType: 'volume',
+          viewportType: 'stack',
           toolGroupId: toolGroupId,
           orientation: viewportOptions.orientation,
           viewportId: viewportOptions.viewportId,