COMP: Fixed warning about signed/unsigned comparison
COMP: Fixed warning about signed/unsigned comparison

Match the type returned by vtkAbstractContextItem::GetNumberOfItems.

Change-Id: I588632253c62f7624b15b189ca1f3a272bd113a3

sumedhasingla/VTK,hendradarwin/VTK,sankhesh/VTK,biddisco/VTK,berendkleinhaneveld/VTK,demarle/VTK,sumedhasingla/VTK,biddisco/VTK,msmolens/VTK,mspark93/VTK,sumedhasingla/VTK,msmolens/VTK,SimVascular/VTK,gram526/VTK,hendradarwin/VTK,mspark93/VTK,demarle/VTK,keithroe/vtkoptix,hendradarwin/VTK,jmerkow/VTK,johnkit/vtk-dev,SimVascular/VTK,gram526/VTK,hendradarwin/VTK,ashray/VTK-EVM,msmolens/VTK,SimVascular/VTK,biddisco/VTK,demarle/VTK,demarle/VTK,candy7393/VTK,gram526/VTK,SimVascular/VTK,keithroe/vtkoptix,biddisco/VTK,sumedhasingla/VTK,msmolens/VTK,jmerkow/VTK,sankhesh/VTK,johnkit/vtk-dev,SimVascular/VTK,demarle/VTK,ashray/VTK-EVM,gram526/VTK,biddisco/VTK,ashray/VTK-EVM,berendkleinhaneveld/VTK,keithroe/vtkoptix,candy7393/VTK,berendkleinhaneveld/VTK,jmerkow/VTK,johnkit/vtk-dev,keithroe/vtkoptix,sankhesh/VTK,sankhesh/VTK,keithroe/vtkoptix,ashray/VTK-EVM,candy7393/VTK,keithroe/vtkoptix,mspark93/VTK,berendkleinhaneveld/VTK,gram526/VTK,johnkit/vtk-dev,johnkit/vtk-dev,sankhesh/VTK,candy7393/VTK,sumedhasingla/VTK,sankhesh/VTK,demarle/VTK,keithroe/vtkoptix,gram526/VTK,berendkleinhaneveld/VTK,jmerkow/VTK,mspark93/VTK,berendkleinhaneveld/VTK,demarle/VTK,jmerkow/VTK,ashray/VTK-EVM,jmerkow/VTK,candy7393/VTK,gram526/VTK,SimVascular/VTK,sumedhasingla/VTK,johnkit/vtk-dev,sankhesh/VTK,msmolens/VTK,SimVascular/VTK,keithroe/vtkoptix,mspark93/VTK,hendradarwin/VTK,johnkit/vtk-dev,mspark93/VTK,SimVascular/VTK,candy7393/VTK,mspark93/VTK,demarle/VTK,berendkleinhaneveld/VTK,msmolens/VTK,ashray/VTK-EVM,hendradarwin/VTK,ashray/VTK-EVM,biddisco/VTK,gram526/VTK,msmolens/VTK,jmerkow/VTK,biddisco/VTK,sumedhasingla/VTK,mspark93/VTK,sumedhasingla/VTK,ashray/VTK-EVM,hendradarwin/VTK,candy7393/VTK,jmerkow/VTK,msmolens/VTK,candy7393/VTK,sankhesh/VTK