correct when InterpolateScalarsBeforeMapping was ON. This was due to the fact
that vtkScalarsToColorsPainter did not consider the log mapping at all when
generating the texture coordinates. Fixed that.

This patch also exposes some static API from vtkLookupTable to make it easy to
compute log values from vtkScalarsToColorsPainter without code duplication
between vtkScalarsToColorsPainter and vtkLookupTable.