Bug 0014446-must enable blending when we use it

Per http://www.vtk.org/Bug/view.php?id=14446 we must be sure to enable blending
before we use it.

Thanks Stephan Rademacher for catching it.

Change-Id: I0678b90bd869579413a6267fece356045c41d305
Merge topic '0014446-raycastrendertexture-must-enable-blending' into master

f9724c30 Bug 0014446-must enable blending when we use it