Fix GRAM size detection under X Window.
Fix GRAM size detection under X Window.

The size in kilobytes was saved in an int which was then multiplied by 1024.
With a board equipped with 2GB, this leads to a overflow and an negative
mem size saved in the vtkXGPUInfo structure.
This bug generated issues with some specific usage of
vtkOpenGLVolumeRayCastMapper for instance.

Change-Id: I54af3bb694e12aaa5b32b70a797920fdada3b309

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