minor backwards compatability change

Change-Id: I50b60dc5c9c538a251163c6bb0c658c58d0113ce
Merge branch 'opengl2-rendering' of https://github.com/cryos/VTK into opengl2-rendering

Change-Id: Id532a7a571bf419359ff466938b9d5d4b5c9f2cf
basic ImageSliceMapper conversion

Change-Id: I630a61d30ac21300afdcc6a97f648b96e2c1cdea