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

Rendering/OpenGL/vtkXGPUInfoList.cxx

@@ -78,7 +78,7 @@ void vtkXGPUInfoList::Probe()
                 ramSize=0;
                 }
               vtkGPUInfo *info=vtkGPUInfo::New();
-              info->SetDedicatedVideoMemory(ramSize*1024); // ramSize is in KB
+              info->SetDedicatedVideoMemory(static_cast<vtkIdType>(ramSize)*1024); // ramSize is in KB
               this->Array->v[j]=info;
               ++j;
               }