vtkOBJImporter: Fix bug in multi-file processing

Static variable was used to vertices indexing.
It causes incorrect start index for other than first file.

(cherry picked from commit 4321af04)

IO/Import/vtkOBJImporter.cxx

@@ -460,6 +460,7 @@ int vtkOBJPolyDataProcessor::RequestData(vtkInformation* vtkNotUsed(request),
     float col[3];
 
     int lineNr = 0;
+    long lastVertexIndex = 0;
     while (everything_ok && fgets(rawLine, MAX_LINE, in) != nullptr)
     { /** While OK and there is another line in the file */
       lineNr++;
@@ -495,7 +496,6 @@ int vtkOBJPolyDataProcessor::RequestData(vtkInformation* vtkNotUsed(request),
       }
 
       // in the OBJ format the first characters determine how to interpret the line:
-      static long lastVertexIndex = 0;
       if (strcmp(cmd, "v") == 0)
       {
         // this is a vertex definition, expect three floats (six if vertex color), separated by