Opacity function doesn't support log mapping.

Since piecewise funciton doesn't log mapping, vtkCompositeControlPointsItem
cannot support showing log-space when in opacity-function mode.

Change-Id: Ib74dddcd347fd49aa2f6bc36cdc9a3daedf6f76c
Merge topic '13082_fix_log_lut' into master

2d67f639 Opacity function doesn't support log mapping.