Save the rendering backend used to the config file
Save the rendering backend used to the config file

This detail is important for projects using VTK, it will also be used
to populate the VTK_LIBRARIES variable with implementations for any
selected modules.

Change-Id: I4f15f9bb221061ab369b5970200617b2dea66b50

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