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
Merge topic 'fix-GRAMdetection-overflow-under-X' into master

6ac4b149 Fix GRAM size detection under X Window.