[vtkusers] vtkVolumeTextureMapper3D has severe performance issue in v6.2

[Weiguang guan]

Hello,

I have experienced extremely slow performance of 
vtkVolumeTextureMapper3D when rendering large volume (LxWxH = 512^3 for 
example). The slowdown issue is new in v6.2, not in the old release v5.1.

Has anyone noticed the issue? What changes has been made to 
vtkVolumeTextureMapper3D between v5.1 and v6.2? Thanks.

Best,
Weiguang