improve "heatmap only" performance
improve "heatmap only" performance

only draw cells and labels that are currently visible on the
screen.

Change-Id: Ic7bfa415952fc9720d199fad790b1a1115746d10

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